Quick Help? Bleeding New Master Cylinder

[mbsl98]

Hi Guys,

Now that I am in the middle of swapping out my master cylinder and ready to start the install, I realize I don't know how to do the initial bleeding of the MC. Past advice was to be sure to "bench bleed" the MC before install, or otherwise I'd never get all the air out later. Now that I have the pieces in hand and have been playing with how they operate, I don't see just how to do the first bleed. Is it just a matter of filling the reservoir and then inserting a rod into the end and cycling the piston a few times with it completely off the car? I don't initially see just how that would get rid of air that would not otherwise just be pumped down through the brake lines to the wheel bleeders, although it would seem to make the bleeding faster to do the first pass without lines attached. If it is just cycling before attaching the brake lines, wouldn't it be just as effective, and easier, to install MC on firewall first, so the cycling was done with brake pedal?

Fast replies very appreciated.

Mike

[Ozark Lee]

There are a couple of methods of bench bleeding and it largely depends on what came in the box. One method is to insert plastic plugs in the brake line side and bench pump until the air stops coming out out the base. The other method is if they sent plastic plugs with tubes that route the brake fluid back into the fluid reservoirs. There you pump until you have a clear stream of fluid with no bubbles.

It is important to bench bleed any new master cylinder, as you said - it will never work correctly unless you do. Bolting it on the firewall (booster) sounds like it makes sense but it puts it at an angle and results in air pockets.

...Lee

[mbsl98]

Thanks. My MC just came with rubber plugs in the two brake line openings. I cut holes in them for plastic tubing that I ran into a jar. Then filled it on bench and cycled it until clear fluid in tubing. Seems to be OK. I put it on the car, and it seems adequate as-is on a trip around the block, without bleeding (At least as good as it was before the change, with completely bled lines). I ran out of light and warm temps for the bleeding and will do that tomorrow. Thanks for the help.