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BRAKE BLEEDING INSTRUCTIONS
CAUTION: Always use the DOT Brake Fluid specified for your vehicle, the most
common are listed below.
DOT3, DOT4, and DOT5.1 are glycol-based fluids, and DOT5 is silicon-based.
One difference is DOT3, DOT4, and DOT5.1 absorb water, while DOT5 doesn't.
Another important characteristics is, as the DOT number raises so does its boiling point.
Although DOT5 and DOT5.1 have similar boiling points, they CANNOT be mixed.
DOT5 is silicone based and DOT5.1 is glycol based and mixing the two will destroy the
seals in the brake system. DOT5.1 can be used as an upgrade or replacement for both
DOT3 and DOT4.
Anytime a brake system is opened to replace components such as calipers, wheel
cylinders, master cylinder, brake lines or hoses, air could get inside. This air must be
removed by bleeding the brakes to achieve a firm brake pedal. Any air trapped in the
lines, calipers or wheel cylinders will make the pedal feel soft and spongy when
depressed. Air is compressible, so any air bubbles in the system will be compressed first.
Then as you continue to apply the brakes, the hydraulic fluid transmits pressure to apply
the brakes. Air in the system takes away from your ability to apply full braking power.
DIAGNOSTICS
Trouble shooting an unknown brake problem, or doing a complete brake conversion or
rebuild. The first thing to look at is the master cylinder. Here are a few symptoms of a
faulty master cylinder.
• Spongy or Unresponsive: Brake pedal that doesn't produce the desired
stopping ability.
• Contaminated Fluid: As the seals inside of your master cylinder wear out,
the rubber crumbles and degrades, often resulting in visual contamination of the
brake fluid.
• Brake Drag: This happens when brakes are applied, but when released the piston
doesn't return fully. Seals have failed and are holding the piston in the pressure position.
You've found that you have a faulty master cylinder, the first thing to do after getting the
replacement is bench bleed it. Once that has been completed and everything installed, then
bleeding off the rest of the system can be done. Bench bleeding the master cylinder is the
starting point. It's simple and best done on the bench, out of the vehicle. There are several
ways to do the job, some involve special threaded adapters for the exit ports that use tubes
that return fluid to the reservoir. Another way is a special syringe that forces fluid through the
exit ports. Using the vacuum pump is the most natural way insuring all the air bubbles have
been removed, and the cylinder is getting completely filled.
BENCH BLEED THE MASTER CYLINDER
Your new master cylinder will come with plugs in the outlet holes, leave them for now.
1. Gently clamp the unit in a vise using the mounting flange with the push rod end slightly
elevated. Do not clamp the master cylinder body or reservoir as severe damage will occur.
2. Fill the master cylinder three quarters full with the vehicles recommended brake fluid
type and, keep it filled at all times during the procedures.
3. Remove one of the plugs from the master cylinder and attach the proper adapter to
the outlet port. Connect the pump tube to the reservoir jar and the jar tube to the
adapter. Connect your vacuum pump to the pump side of the reservoir jar.
4. Operate the pump and observe air and fluid flowing into the reservoir until clear.
Continue pumping until a bubble-free fluid appears. It may take 4-5 pumps before that
circuit is full and free of air.
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