The master cylinder is a foot operated hydraulic pump that sends pressurized brake fluid through the brake lines and into the brake calipers/wheel cylinders.
In its simplest form, the master cylinder consists of a housing, reservoir, piston, rubber cup, return spring and a rubber boot (manual only).
A cylinder bore is machined into the center of the housing. The spring, cup and piston(s) slide in this bore and produce hydraulic pressure. The reservoir keeps the system full of fluid as the brake lining wears and the dust boot serves to keep contaminates from entering the rear of the master cylinder.
The dual master cylinder is required on all vehicles manufactured since 1967. Dual master cylinders have two separate chambers that separate the front and rear brake circuits.
This type of system prevents the total loss of braking action in the event of brake fluid loss. The brake circuits can be split front and back or diagonally. Both styles will stop the vehicle with only one circuit operating, but it is not safe for normal use.
Operation of the master cylinder is simple. When the brake pedal is depressed, force is applied through the push rod to the master cylinder piston.
As the piston cup is forced forward, brake fluid is trapped and hydraulic pressure builds. Hydraulic pressure causes the movement of brake assemblies, such as calipers or wheel cylinders, to stop the rotation of the wheels. When the brake pedal is released, fluid is forced back through the lines into the master cylinder reservoir.
The most common cause of master cylinder failure can be attributed to:
Contamination - brake fluid is hygroscopic (attracts water)
Normal Wear
External corrosion
The most common problem customers face when installing master cylinders is improper bleeding procedures. In the past, some vendors supplied a bleeder kit that consisted of plugs with holes.
Tubes pressed on to the holes in order to bench bleed the master cylinder. Although this kit was successful when used properly, it was not the best process.
One of our goals in Technical Services is to improve every process. We found a better way.
The new bleeder kit comes with solid plugs to bench bleed the replacement unit.
The advantages of the new process are:
More efficient to bleed the master cylinder.
No mess or fluid dripping when bench bleeding or when installing the unit on the vehicle.
Proving the unit is functional during bench bleeding. If the piston is firm when bench bleeding and not on the vehicle when bleeding, the brake system is at fault, NOT THE MASTER CYLINDER.
The following process is supplied with replacement master cylinder. You can help our customers by stressing the importance of following every process we include with our parts, to prevent warranties and increase sales.
1) Transfer proportioning valves, switches or other components to the replacement master cylinder (as applicable).
2) Tighten bleeder plugs into ports (bleeder plugs may be in parts kit or master cylinder reservoir).
3) Support the master cylinder in a vise in a level position. Never clamp onto the body of the master cylinder.
NOTE: Units with remote reservoirs should be bled on the vehicle. It is VERY IMPORTANT that the body of the master cylinder be kept level during the bleeding process.
4) Fill reservoirs halfway or until reservoir ports are covered. Always use new brake fluid from a sealed container as specified by the vehicle manufacturer.
5) Install reservoir lid so brake fluid does not spray from reservoir during bleeding process.
6) Using a blunt tool or dowel (brake pedal if mounted in vehicle), SLOWLY press in and release the master cylinder piston using short strokes of 3/4” to 1”. NEVER STROKE PISTON MORE THAN 1”. Repeat this step until resistance to piston movement is firm and less than 1/8”.
IMPORTANT NOTE:
On step-bore master cylinders, WAIT 20 SECONDS BETWEEN STROKES. This will allow air trapped in the quick take-up valve to rise into reservoir. MASTER CYLINDER BODY MUST BE LEVEL.
7) Install master cylinder on vehicle.
DO NOT remove bleeder plugs until brake lines are ready to be installed. This prevents air from entering the unit and brake fluid leaking out.
8) Remove one bleeder plug at a time from the master cylinder and connect the proper brake line to that port. Repeat this step for remaining brake line ports.
Be sure fluid level is maintained during installation of brake lines and that all fittings are clean before installation. Bleed brake system according to vehicle manufacturer’s procedures and specifications.
9) Verify proper pedal action before moving vehicle. Do not road test vehicle until correct pedal feel and travel are obtained.
NOTE: If firm brake pedal action results using bleeder plugs, but becomes soft after installation in vehicle, the problem is not the replacement master cylinder
