Grain truck master cylinder - GMC 6500

andy r · Jul 3, 2017

I was hauling some soybeans to town today and my brakes seemed spongy and the brake fluid would pretty quickly disappear. Truck is a 1979 GMC 6500. Brakes consist of a master cylinder piped to a vacuum booster. The master cylinder is just a simple one - bleeder on top, pipe to the booster down behind the wheel well. What I found was brake fluid coming out of the bottom of the master cylinder from a little "dimple" in the casting on the underside. I didn't have anyone around to push the brake pedal, but I assume it would possibly come out with force if the brake pedal was pushed. The way it was it would just drip every 15 seconds. Is the master cylinder shot and allowing brake fluid past the cups inside??? Can't recall ever seeing a master cylinder with the hole underneath. Thanks for helping out.

Steve@Advance · Jul 3, 2017

Sounds like a "Hydrovac" brake booster.

Best to replace the whole thing with a rebuilt one.

Might take a look at the master cylinder, pull the boot back under the dash. If wet, be a good time to replace it too. Don't want to contaminate the new rebuild with dirty fluid from the old master cylinder.

And if any brake lines are needing replaced, or any wheel cylinders leaking, best to get everything working at once. The hydrovacs can be a bear to bleed.

Janicholson · Jul 4, 2017

If the leak is in the casting, there is no choice but to replace it. Use a mirror and a clean dry rag to be sure it is the leak you think it is. Brake fluid is able to "wet" metal surfaces with ease, and can originate elswhere. The hydrovac unit may be just fine. Lets hope. Jim

John B. · Jul 4, 2017

Your master cylinder should not be leaking. Be sure to check all the brake lines also. Usually on older vehicles a metal brake line will rust out and leak. If that happens you just as well replace all of them because one new line causes the next weaker one to rupture. Just speaking from years of experience. One time I had to make a service call to a farmer saying his brakes went out on his Ford 600. We found the bad line under the cab and he said we'll just put a peice of rubber hose with clamps on it. I said what? He said yeah a hose with clamps that will get me by until I have time to fix it right. I said no way. I then asked him if he knew how much pressure is produced in those lines. Hes said I guess about 100 psi... I said try 2000-3000 psi. He shut up then and let me replace the line.

Joe(TX) · Jul 6, 2017

I agree with Steve. If the booster is leaking, brake fluid gets sucked into the engine. A good way to tell is if the engine speed changes when you appl the brakes.

Grain truck master cylinder - GMC 6500

andy r

Member

Steve@Advance

Well-known Member

Janicholson

Well-known Member

John B.

Well-known Member

Joe(TX)

Well-known Member

Similar threads