Bench Bleeding Notes

BENCH BLEEDING

Why
A master-cylinder will develop little or no pressure if air is trapped in cylinder bore. Normal brake bleeding will not remove air trapped within the master-cylinder.

How
Clamp the master-cylinder securely in a vice. It is usually best to clamp the side of the vice onto the flat surface that is used to secure the master-cylinder to the brake booster.

There are two basic methods for bleeding master-cylinder, one utilizes stroking the master-cylinder to expel air from the cylinder bore while the other utilizes a large syringe to backflush fluid from the outlet ports to the reservoir.

Stroking
- Fill the master-cylinder with clean brake fluid.
- The brake line fittings on the master-cylinder should be capped or plugged.

NOTE: Special "bench bleeding" plastic fittings are available which allow for recirculating the brake fluid back into the reservoir.

- Using a dull object, slowly stroke the master-cylinder piston, air and brake fluid will be expelled from the brake line fittings. Prior to releasing the piston plug or cap the outlet ports.
- Repeat this procedure 8-10 times or until no air is emitted from the outlet ports.
- When finished, cap the outlet ports and install the master-cylinder.

Syringe
- Fill the syringe (one especially designed for brake bleeding) with clean brake fluid.
- Insert the end of the syringe into one of the outlet ports on the master-cylinder.
- Slowly compress the syringe and back flush the brake fluid through the master-cylinder.
- A combination of air and brake fluid will be emitted from the inlet port in the fluid reservoir.
- Repeat this procedure until only brake fluid is emitted into the reservoir.
- Cap the outlet ports and install the master-cylinder.