Diagnosing a soft brake pedal can be a difficult task. Paying close attention to the details will help reveal the problem areas. Some of the most common mistakes made by technicians when performing this type of diagnosis are:
• Having problems getting a pedal back after doing brake work;
• Over-adjusting the rear brakes to fix a low brake pedal;
• Using too much brake fluid;
• Returning a master cylinder as defective;
• "Pumping" up a vehicle with a quick take-up master cylinder; and
• Bleeding only the front wheels on a vehicle equipped with a diagonal split hydraulic system.
We are going to take a very methodical look at what causes low brake pedals and how to accurately diagnose and repair the problem. In addition to low pedal problems, we will also be discussing spongy brake pedals. Let us begin by breaking down the major categories that contribute to causing a low or spongy brake pedal:
• Air in the system;
• Mechanical problems in rear drum and rear disc brakes;
• Mechanical problems in front disc brakes;
• Power booster and/or pedal linkage problems;
• External hydraulic leaks;
• Internal master cylinder failure; and
• Miscellaneous causes — valving, brake hoses, etc.
With all things considered, the list of possible causes is quite a long one. Before we look at this list, we need to differentiate a low pedal from a spongy pedal. Many times the descriptions are used to explain the same condition. For the purposes of this article, a low brake pedal will be defined when the brake pedal is applied and it is considerably lower than it should be. Stroking the pedal will bring up the pedal height and the pedal feel will be toward the firm side. A spongy pedal will also be low and will pump up. The main difference is, even when it's pumped up, it will have some sponginess to it. There are cases where the two conditions exist on the same vehicle, but generally it is one or the other causing the primary problem.
Now that we have that out of the way, where do we start? How about at the heart of the matter, the master cylinder. According to many of the master cylinder manufacturers and remanufacturers, many shops have claimed a master cylinder had a heart attack but after the autopsy, nothing was found wrong with the heart. Only about 5 percent of the master cylinders returned as defects actually have a problem with them. It is this writer's opinion that the master cylinder is the most misdiagnosed part on the brake system. According to one national brake hotline, the record for one vehicle is nine master cylinders. If we are going to be able to get to the root cause of a low-, spongy-pedal complaint, then we need to clear this up.
The number one reason master cylinders are misdiagnosed so often is because not everyone has a good enough understanding of how master cylinders work to diagnose them properly. Don't take offense, I'm simply stating my opinion based on what I see in the field and what I hear talking to other people who are close to the issue. Read on and form your own opinion.
A master cylinder is a specialized pump that pressurizes the brake fluid when the brake pedal is applied. The amount of pressure produced is determined by the amount of pedal force applied. The parts responsible for this inside the master cylinder are the primary cup seals. There are two primary cup seals as shown in Figure 1. When the brake pedal is applied, the primary cup seals are pushed forward, causing the lips of the seals to be pressed against the cylinder bore. The harder the pedal is applied, the tighter the seal.
The last time a customer came in to your shop complaining of losing the pedal, how did he or she describe it? Chances are it was described as, "I was sitting at a stop light and all of a sudden, my brake pedal went to the floor." Talking to the customer further reveals that it only happens every so often and seems to be happening more frequently now. Why at a stop light and not during a stop and why is it getting worse?
Understanding what takes place inside the master cylinder makes the answer to these questions easy. When at a stop, the cup seals are not being pressed against the bore very hard because not very much pedal effort is required to keep the vehicle from moving. Any defects in the lip of the seal or the cylinder bore will show up in light pedal application situations like this. The reason it is happening more frequently is because the defects are getting worse. So, intermittent pedal loss is almost always a bypassing cup seal in the master cylinder. It is also a sign of impending master cylinder failure. When diagnosing this problem, use light pedal force to duplicate the problem.
What about if you see a dampness where the master cylinder bolts to the vacuum booster? What does this mean and will it affect the brake pedal? Fluid loss out of the front of the master indicates a leaking secondary cup seal. (See Figure 1.) The secondary cup seal's function is to keep the fluid inside the master cylinder. Figure 2 shows the fluid path when a leak occurs. You can see the fluid leak is supplied by the reservoir. The leak is fed by gravity. This means unless the reservoir goes dry, it will have no affect on the brake pedal. So if you have a low pedal and a leaking secondary cup seal, don't tie the two together as cause and effect. Finish the diagnosis to find the real cause.
What about the master cylinder causing a spongy pedal? Many master cylinders have been replaced because they seemed to be causing a low, spongy pedal. Going back to our earlier discussion, a master cylinder is a pump. It either works or doesn't. And when it stops pumping, you get intermittent pedal loss. Conventional master cylinders will not cause low, spongy pedals. On the other hand, air in a master cylinder will cause a low, spongy pedal. Most, but not all, master cylinders are designed so the outlets of the master cylinder are at the highest point as shown in Figure 3. Any air in the master cylinder can exit out to the wheels. There are a number of master cylinders that do not fall into this category. These master cylinders have the secondary outlet away from the end of the bore as shown in Figure 4. To complicate this, the master cylinder is mounted at an angle, which causes any air in the secondary bore to be trapped. We will discuss this issue in more detail later in the article. For now, understand master cylinders do not cause spongy pedals!
There is one last issue we need to discuss in regards to master cylinders. The primary cup seals spend most of their lives in a certain portion of the bore. This area stays clean and polished because of this. The remainder of the bore becomes covered with sludge, corrosion and rust as shown in Figure 5. This doesn't create a problem unless the seals are pushed into this corrosion. If this happens, the seals may become damaged. This usually can happen during a hydraulic failure or during brake service. If a vehicle enters your shop with a hydraulic failure, you need to either determine if the master cylinder is good or leave the door open with the customer. For example, if the rear axle line on a RWD vehicle failed, you could bleed the system using a line lock on the rear brake hose. If the pedal comes back, then the master is good. If not, add it to the estimate. If this isn't possible, use a plug or dummy line in the failed circuit to isolate it. Bleed through the plug or dummy line and determine the master's condition. During brake service make sure anybody stroking the brake pedal, either during bleeding or pumping the pistons out, knows not to exceed three-quarters pedal travel. To be on the safe side, use a 2x4 under the brake pedal. Not much is worse than a no-pedal-after-brake-service scenario.
Now that we have the master out of the way, let's get into the heart of the matter: what to do when the cause of a low brake pedal is not obvious. The most valuable tool to have in your possession when diagnosing a low, spongy brake pedal is a good set of line locks. There are a number of different styles available, but one stands out above the rest. The line lock shown in Figure 6 is the best type to use when applying pressure to the system during the diagnostic process. Plier-type line locks have a tendency to bleed off easier when pressure is applied to the system. This will lead to misdiagnosis if not caught during testing. The line locks will enable us to take a rather complicated brake system and break it down into smaller, more manageable chunks.
Before using the line locks, there are a couple of things to check. First, see if the pedal pumps up and try to determine if you are dealing with a low or spongy pedal. If you suspect air in the system and the vehicle has an open-face-style reservoir, you can do a quick check to see which circuit the air is in. Drain about half the fluid from both reservoirs. Have an assistant pump the pedal up and hold pressure on it. Observe the reservoir while your assistant quickly releases the brake pedal. If there is air in the system, it will have been compressed during the pumping. Now that the pressure is released, the air will expand and push the fluid out of the system through the vent port. This will cause a mini-geyser in the reservoir. If no geyser is seen, chances are there is no air. If air is suspected, or you can not perform this test, you may want to consider bleeding the system out before continuing. This is especially true if the pedal problem is related to work having been just done on the system. Bleeding is covered at the end of this article.
Now we are ready to pinpoint where the low or spongy pedal is coming from. Before we install the line locks, a couple of words of caution are in order. First, only use approved line locks. Vise grips and other such tools can damage the brake hose internally. Some newly designed hoses can also be damaged internally by line locks. Second, don't use line locks on severely deteriorated brake hoses.
At this point let's assume we have a vehicle where we can use line locks. Install line locks on each brake hose in the system. Before applying the pedal, one more word of caution is needed. It deals with potential master cylinder damage because of the use of line locks. Take a close look at Figure 7. Observe how close the cup seal is to the vent port. If we install line locks on this system, we eliminate all mechanical movement. The piston will move only slightly when the brake pedal is applied because there is nowhere for the fluid to go. The cup seal is forced up into the vent port and gets ripped. We can prevent this by adding one step to our process after the line locks are installed. Loosen either line at the master cylinder and have an assistant depress the brake pedal about 1/2 inch and hold pressure on it. This moves the cup seals past the vent ports and will prevent any damage.
The line locks break the system down into either four or five parts based on what type of hydraulic system we are working with. On a front-to-rear split system, we have both fronts individually, the rears together and the rest of the system from the line locks up to the master cylinder. On a diagonal-split hydraulic system, we have the same except the rears are individual. Before continuing, we need to make sure we have an accurate test. While still holding pressure to the system, try to rotate each tire. All should rotate freely. If any of the line locks have bled through, you need to start over. Only continue after all wheels have been isolated.
Before discussing the possible results of the test, let's look at what we just did. Normal pedal travel on a vehicle is about 1 inch. What causes this? In the rear brakes, it is the gap between the shoes and the drum. In disc brakes, it is the gap between the disc pads and the rotor. Each accounts for about 1/2 inch in most vehicles. By installing the line locks we have eliminated this movement. The brake pedal should be rock hard. If there is a problem in front of the line locks, then there will be pedal movement. It will not be the same as with the line locks off because this adds normal front disc and rear drum movement.
LINE LOCKS INSTALLED AND PROBLEM IS STILL PRESENT
If the pedal is low or spongy at all with the line locks installed, there is a problem in front of the line locks. If we short list the possible causes, we come up with the following:
• Air in the system in front of the line locks;
• Bypassing cup seal in the master cylinder;
• Pedal linkage problems or adjustments;
• Booster pushrod adjustment; and
• Valve failure (only applies to certain types of ABS valves).
Problems with the pedal linkage or booster pushrod should be easy to spot. If either is adjusted so they are "too short," free play will be felt in the pedal. There will be little or no resistance until the pushrod contacts the master cylinder piston. On vehicles with adjustable linkage, make sure to follow the manufacturer's procedures. On vehicles with adjustable pushrods, use a tire crayon to adjust the gap between the pushrod and piston. To do this, place a small piece of tire crayon on the end of the pushrod and hold the master cylinder against the booster until it's flush. Pull the master away and check the crayon. If it is crushed, then the rod is too long, if untouched, it is too short. If you leave the rod or linkage too long, you will create a new problem because the vent port will be covered. No venting will take place and a drag condition will be created.
We have already discussed the bypassing cup seals. We would be inclined to investigate this further based on the customer's description of the problem. Although valve failure is listed, I want to clarify something. Conventional brake valves such as metering and proportioning valves do not fail in a way that would cause low or spongy brake pedals. They can, however, have air trapped in them that will contribute to a low or spongy pedal. This will be covered later. The valves that can cause a pedal problem are the RWAL, RABS and 4WAL valves. These valves all have low pressure accumulators and dump valves. The dump valves can leak and allow fluid into the low pressure accumulator during normal braking. This will cause a drastic loss in pedal height. Because of this, the first thing that should be checked is the dump valve on vehicles with RWAL, RABS or 4WAL that are experiencing a low, spongy pedal. For detailed steps on how to do this, see the article, "Diagnosing Ford Brake Systems," in the June issue of Brake & Front End.
That leaves us with air. If air is causing the low-pedal condition with the line locks installed, chances are it is trapped in one of the components. The list would include the master cylinder, brake valving or ABS modulator. We have already discussed master cylinders where the outlets are not at the highest point. Figure 8 shows another example of this. This is a General Motors Quick Take Up master cylinder. It is used on GM FWD vehicles from 1980 to about 1987. This master cylinder has an integral pressure differential switch. The top of the bore around the plug for the pressure differential piston is the highest point in this master cylinder. Any air in the secondary circuit, which supplies the left front and right rear wheels, will settle here. This will cause a right front premature wear problem as well as a low, spongy pedal. There are two methods to get this air out. One is to raise the rear of the vehicle to make the master more level in hopes the air will be able to be bled through the line. Care must be taken so the vehicle is not lifted in an unsafe manner. The second alternative is to bleed the master through the 5/16-inch hex plug. Care should be taken when loosening the plug so the O-ring isn't damaged. Work the plug back and forth after squirting a little brake fluid around the plug. Bleed using the single stroke method until no air is seen.
Certain conventional brake valves are prone to trapping air. The valve shown in Figure 9 is one such animal. This valve fits most Chrysler FWD vehicles made in the '80s and early '90s. It is a combination valve that houses a pressure differential switch and two proportioning valves. If air is allowed in either rear circuit, it can become trapped around the spring on the proportioning valve as shown in Figure 10. If after you have serviced a vehicle with this design valve you still have a low, spongy pedal, then you should bleed this valve. The best way to do it is to bleed it using the proportioning valve retaining nuts. Loosen the nut until fluid flow is established. Use single stroke bleeding combined with tapping on the valve housing with a brass hammer. Repeat until no air is seen and then bleed the remaining nut.
LINE LOCKS INSTALLED AND PEDAL IS HIGH AND HARD
Most of the time after installing the line locks, the pedal will be high and hard. This indicates that the problem is at one or more of the wheels. With pedal pressure still applied, remove the rear line lock (on RWD vehicles). Remember some pedal drop will be normal. If most of the pedal drop comes back after releasing the rear line locks, then the problem is in one or both of the rear brake assemblies. The most likely cause will be a non-functioning self-adjuster on one or both rear wheels. It could also be that the process that performs self-adjusting is not being done by the customer. Most FWD vehicles use the parking brake to accomplish self-adjusting. How many customers religiously use their parking brake?
Before we go on, we should talk about a very bad habit many technicians have. Many technicians automatically adjust the rear brakes to "cure" a low brake pedal. Many of them also think this is a legitimate fix because when they try the pedal, it is better. However, the problem is that when rear brakes are over-adjusted it can hide the true cause of the pedal problem and create new problems.
Rear brake adjustment should only be done after removing the rear line lock, duplicating the pedal problem and it is found to be a problem with the self-adjusting mechanisms. When rear brake adjustment is required, there are two acceptable methods. The first is to use the factory procedures that, at times, are very involved. The second is to use a combination of a quick set to rough the shoes in and then let the vehicle's self-adjusting mechanism do the rest. On a duo-servo, back up and make several stops to complete the process. On most FWDs, apply and release the parking brake six to 12 times. We, as an industry, have to get it out of our heads that rear brakes should have a drag on them when adjusted properly. This is not correct.
Let's assume that after we pulled the rear line lock that only normal pedal drop was felt. This would mean that the problem is in either one or both front wheels. Release one front at a time, noting pedal drop. The results will be that either one wheel is accounting for most of the problem or each front is responsible for half the problem. Most of the time, the problem will be related to excessive mechanical movement. Remember, the caliper pistons are much larger than the master cylinder piston. It takes a lot of pedal travel to move the caliper pistons only a small distance. Here is a list of possible causes:
• Air at one or both of the front calipers;
• Seized slider mechanism;
• Binding or cocked brake pads;
• Excessive wheel bearing play;
• Excessive rust build up between pad/shims and caliper;
• Self-adjustment is not being allowed in the caliper(s); and
• The vehicle is equipped with low-drag calipers and has a failed QTU valve.
Our inspection would include a quick check of wheel bearing play, bleeding the calipers and rechecking the pedal. If the problem is still present, watch closely as the pedal is applied and released. Watch for normal caliper movement and see how far the piston is moving. A seized slide will cause only the inboard pad to apply. This can actually cause the rotor to be flexed during application. This causes excessive piston movement, which creates the low pedal. The same is true if the calipers are not self-adjusting when they are supposed to be. As the pads wear, the piston is supposed to move out in its bore. If the seal is gripping the piston tighter than it should, it will hang on to the piston longer. It acts like a low-drag caliper. Excessive corrosion in the square-cut seal groove and excessive corrosion on the piston sealing surface can cause this to happen. In addition to these checks, a close visual inspection is in order. Pads that fit onto rails can become bound up or be installed incorrectly. Either of these will cause excessive piston movement.
Another often overlooked cause is excessive rust build up between the pads/shims and caliper. As this rust grows, it forms a compressible layer, which ultimately results in a low pedal. You should also check to make sure the calipers have been installed on the right sides. Don't laugh — it happens and it causes a great deal of aggravation. I have seen entire shops stumped by this very same problem.
If a vehicle equipped with a QTU master cylinder had a defective QTU valve and line locks were installed on the front hoses, what would the pedal feel like? The answer to this question is high and hard. Why? Because the reason the vehicle has a QTU valve is the low-drag calipers. Take them out of the picture and it doesn't matter if the QTU valve is working or not. So how do you diagnose whether the QTU valve is working? First you must identify if the vehicle has a QTU master. A QTU master will have a step bore as in Figure 11. (The Volvo step bore master cylinder is the only exception to this rule that I am aware of.) If the vehicle has a QTU master and the low-pedal problem is related to the front brakes, the QTU valve must be added to the list of possible causes. There is no effective method to field test the QTU valve so you have to use a process of elimination. Verify proper front brake operation first. If the problem is still present, go after the QTU valve.
PROBLEMS GETTING PEDAL BACK AFTER SERVICE
Many times a vehicle enters the shop with a perfectly acceptable pedal only to have little or no pedal after brake service has been performed. This is especially true when the brake service includes hydraulic service. There are ways to minimize the chances of this happening without shortchanging the quality of repairs. The process would include:
• Using a line lock on the hose of the component being changed;
• Changing the component;
• Removing the old fluid from the reservoir and replacing with new;
• Gravity bleeding the system at each wheel; and
• Bleeding each wheel in proper sequence with an approved method until no air is seen and the fluid is clean.
Using this process will generally yield a nice high, hard pedal.
Deviating from the steps listed, skipping steps or making up things as you go along is where the problems start. A common mistake is to use the wrong bleed sequence for the vehicle. RR, LR, RF, LF and RR, LF, LR, RF are not universal. There are 32 base-bleeding sequences with many having added steps for specialized valving and ABS modulators. And the rule of thumb, longest first, doesn't apply anymore. There are vehicles that start with the left front wheel. The days of pulling this information out of your head are gone.
The method you use to bleed can play a part in whether you encounter additional problems. If you are still foot bleeding, you should be using the single-stroke method. Open the bleeder, push the pedal down three-quarters of the way and close the bleeder. Pumping the pedal up or surge bleeding runs more of a risk of damaging the master cylinder. Some master cylinders can be damaged internally by surge bleeding. If you surge bleed a QTU master, be prepared to be there a while. You will spread any air out in tiny little bubbles that are very difficult to remove. QTU vehicles use manual bleeding with five to 15 seconds between strokes.
If you vacuum bleed, you have probably run into "false air." This is air that is sucked around the bleeder screw. It makes it difficult to know when you're done and makes the process less effective. This is so true that GM no longer endorses vacuum bleeding on its vehicles. If you are going to use vacuum bleeding, try using Teflon tape on the bleeder screws. Leave the line lock on the wheel until you are ready to bleed and position the bleeder hose straight up so it has to fill up. This will yield the most effective vacuum bleeding.
Pressure bleeding is by far the most effective one-man bleeding method available. There are a number of one-man pressure bleeders available today. Anything from the well-known tank bleeder, to one that connects to your spare tire, one that uses a process similar to a bug sprayer and one that uses a hand pump. Each has its own set of advantages and disadvantages.
What happens when you do run into a low, spongy pedal after bleeding the vehicle out? What do you do now? Knowing how to deal with this scenario and having the tools to do it is critical to your success. You can use some of the techniques we have discussed to pinpoint the source of the problem, but you still need to get the air out. Conventional bleeding techniques, combined with steps like tapping the component while bleeding, may yield the desired results but there is no guarantee. A new process was developed three years ago called RFI™ (Reverse Fluid Injection™
. In this process, fluid is injected into the bleeder screws to force the air to the master cylinder. (See Figure 12) At first there was a wide-spread concern about pushing contaminates upstream to the brake valving, ABS modulator or master cylinder. RFI™ has been shown not to run this risk. The reason is because it is only done after the system has been flushed and it works at very low pressures. However, there is one thing to watch for when using RFI™ on a vehicle with an angled master cylinder. If you use RFI™ without doing anything about the angle, you will force the air into the end of the master, not the vent port. This will not fix anything. Either raise the rear of the vehicle so the master cylinder is at least level without causing a safety problem or remove the master from the vacuum booster and position it so the front of the master (input side) is at the highest point.
