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Brake master cylinder leak/booster question

The brakes work fine with two rebuilt calipers just installed, but I noticed a little seepage of brake fluid from the base of the master cylinder. The black paint on the power brake booster, just under the MC, is damaged. The only times I’ve added fluid are when bleeding the system after brake work, several times over the years, and when a brake line to the rear wheels was leaking. So, not much of a leak from the MC, but still…

If I install a new or good used master cylinder, what if anything needs to be done about the booster? Maybe some brake fluid is in it. How big a problem would that be?

It’s a 1999 Civic EX with 4 wheel ABS.

I’ve replaced many brake master cylinders where they leaked brake fluid into the vacuum booster.

I use a suction gun to remove as much fluid as possible before installing the new MC.


So, probably no damage done as long as I get the fluid out?

It’s probably such a small amount that hardly any has pooled in the bottom of the booster - but I’ll follow your suggestion to try pulling any out.

You should get a brake warning light if there is a leak somewhere. Does the brake pedal feel good,nice and firm?Do you have to top up the brake fluid reservoir often?
My 99 Corolla has a little bit of missing paint on the base of the booster right under the master cylinder because I must have spilled some when I bled the brakes.

Well, that might be the source of my problem. Brakes work fine. Maybe do nothing for now.

One related problem is I melted the side of the MC cap with a shop light recently, and bought an aftermarket one that does not fit snugly. So I plan to get a cap from an auto recycler nearby - and maybe just get the whole MC.

You should be able to get the original cap from a wrecker yard…they made so many of these cars.The brake fluid could be slushing around and escape by that tiny opening.

That “tiny opening” is the air vent to the back side of the diaphragm that allows the diaphragm to extend down into the reservoir as fluid is drawn into the system from the MC. No amount of sloshing should produce fluid from that minuscule hole unless the diaphragm is also damaged.