hissing noise from master cyl?

[goertz]

ok, in traking down my various vaccuum leaks, i found the strangest thing i've ever seen in my 20+ years of tinkering with GM cars. there is a vaccuum leak from the master cyl. NOT THE BOOSTER, the master. on the bottom of the master, near the flange, there is a 3/8 horizontal hole that goes through the casting, and a ton of vaccuum is leaking through there. so i figure, i'll put a bolt through the hole, to see if that fixes it. well the vaccuum leak stops, but so do the power brakes. anyone else ever experienced this? maybe the booster is bad, and leaking through this hole. maybe the hole is some sort of fluid drain just in case something got into the booster? what gives?

 

[lone73]

Sounds like the booster is leaking at the front. Haven't had mine off in a while. I can't remember if there is supposed to be a seal there between the booster and master.

I'm guessing you have a booster problem though, not a m/c prob. There is no vac to the m/c.

good luck
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[srs244]

there is definitely a seal on the booster between the booster and master cylinder. when i had to replace my master cylinder when the piston started to allow fluid to pass behind it, i tried to save the brake booster (equally as old), but the seal just fell apart and wasn't salvagable. i decided to just replace the whole system and be done with it at the same time.

your problem has to be booster related if you are convinced that the vacuum leak is originating from there as the master cylinder does not have any vacuum to or from it. it is purely a hydraulic system.

 

[goertz]

right! I am well aware there is no vaccuum going to the master, which floored me when i could feel strong vaccuum coming from that hole in the master. i thought maybe there should be a seal there too, but i think the booster is just no good. my guess is that hole in the master leads into the booster to allow the booster to return after it's depressed. if it was air tight, and you applied brakes, it would stay applied without some way to let air back in. but i'm sure it shouldn't have vaccuum coming out of there. maybe the diaphragm in the booster is torn, allowing vaccuum to escape. i figured if i plugged the hole, all would be well, but it seems like it has NO power brakes at all when that hole is covered. althought the vaccuum leak is eliminated, and the engine idles much smoother.

 

[JohnZ]

The thick cup-type seal between the master cylinder and the booster is essential in order to retain vacuum on the front side of the diaphragm; if that seal fails or deteriorates, you'll have a vacuum leak. The pushrod from the booster to the back of the master cylinder also has an O-ring on it inside the booster to seal vacuum where the pushrod passes through the center of the cup seal. The outside air entry into the booster's power valve is through the back of the booster, where the pedal pushrod comes out - there's a foam dust seal there to keep dirt out and to muffle the sound of the air entering the booster every time you step on the brakes. There is no vacuum in the master cylinder casting (unless it's cracked or corroded internally).

 

[goertz]

ok, found the seal inside the booster between the master and booster, and it's all torn up, which causes the vaccuum to leak past the booster, and through the weephole in the bottom of the master. now, to just find the replacement part, that should be fun!