I ordered new calipers and a master cylinder for my '81 Vette and read in other threads that the master cylinder requires bench bleeding.

Since this is my first time that I install the master cylinder, can somebody describe what is involved, how to do it and if I need additional tools such as vacuum pump etc....

My parts will hopefully arrive tomorrow (Thursday) and everything must be ready by Saturday evening since I have to leave on a 4 week business trip on Sunday morning.

Thanks in advance

Roger

 

I just did mine a couple of weeks ago. I stuck it in a vise with a rag around it, 1/4 filled it with fluid and pushed the rod in with a screwdriver (blunt). I pushed with one hand and blocked the outlets with my fingers on the return stroke. It will take a few primes and only travel about 40mm.

When the fluid comes out it really comes out! So use the rag, and leave enough fluid in there so it doesn't suck air.

This may not be the "correct" way, but it worked for me.

 

When you get your master cylinder, it should come with a couple of small tubes and fittings .
The fittings and should go into where the brake lines would go. Put the tubes on the fitting and route them back into the master cylinder. Fill the master cylinder with fluid, and then using a screwdriver or a socket extension pump the master cylinder very slow. As you pump the cylinder, you should see some air bubles in the lines, continue to pump the cylinder several times, and the air bubles should go away. Your cylinder has been bleed.

 

Not all replacement master cylinders come with the fittings & tubes. This depends on the source. I just bought one last week that was sealed/unopened and the box did not have the fittings or the tubes. These can be purchased separately, or you can make them out of replacement brakeline from an autoparts store.

The objective is to get all of the air out of the master cylinder, replacing it with fluid. The purpose of the tubes is to provide a way to route the expelled fluid back to the reservoir as you hand "pump" the cylinder. Pushing in on the piston will expell fluid. On the way back out, the system will attempt to suck air back into the cylinder through the port(s).

In this process, it is absolutely critical that you do not let air back into the cylinder on the "up-stroke" (release) of the piston. The tubes provide this function as well, but ONLY if you keep them submerged while activating the piston, and only if you expell all of the air out of the tubes initially. It is also critical that the fluid in the reservoir never get down to the tiny orifice in the bottom of the reservoir. If this happens, you must start all over again or you will end up with a "spongy", but functioning system.

The same result can be obtained by holding your finger(s) over the ports and letting the fluid out, but not let air back in. Unless you rig up something to catch the fluid without contaminating it, you will sacrifice brake fluid using this method. If it's not $20. / quart silicone, no problem. The tiniest spec of dirt can ruin the seals in a master cylinder. You must also try to make sure that no fluid leaks out after you remove your finger. This can result in air getting back into the cylinder. However, this can correct itself when you bleed the brake lines.

The master cylinder can be "bench bled" while on the car using the tubes. Using your fingers, you will need a helper to press the pedal, or, you need to remove the cylinder and bleed it on the bench. The idea is to follow the procedure to get the air out of the chamber.

Hope this helps.
Tom

[Modified by Tom454, 9:28 AM 8/9/2001]


[Modified by Tom454, 9:32 AM 8/9/2001]

 

If your mc does not come with the bleeding kit, you can get one from NAPA for just under $7.00.