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Discussion Starter · #1 ·
I just noticed a tiny piece of black tape under my master cylinder today, so i took it off, but when i started up my truck i heard a hissing noise, kind of like a vacuum leak, and sure enough there was a tiny hole that the tape was covering up, maybe 1/8" in diameter, undernieth the master cylinder at the back, by the brake booster. Does anyone know what that is for? Can i just plug it? Ive been driving it for years and years and i dont remember putting the tape over the hole. everything seemed fine.
 

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Hi Rob
I'm not sure why there is a vacuum leak in your master cylinder, were all your vacuum should be contained in your booster and any whole in your master at the least may leak fluid if the whole enters the bore. Plugging the whole may be best, if the tape has been there for all these years. Pictures would really help. What year truck are we talking about ? This truck isn't equipped with a hydro vac system by chance ?
 

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If its (the hole) on the booster then it would be a vac. leak never heard of putting a hole in the booster on purpose. Maybe the seal on the booster is leaking. As Steve said a picture would help a bunch. Or just replace the tape and keep driving :D :D anim_25
 

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Actually thinking about this more the diaphragm in the booster is separate from the container, if it is leaking out the metal part the diaphragm is bad. that hole may be something put in at the factory to let you know you have a problem. Pheew its 16coffee time now
 

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I`m thinking that someone at one time tried to screw something to the booster and created the hole before they realized their mistake.
 

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Now I`m thinking the rubber seal at master / booster, I had one leaking there.
 

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Discussion Starter · #10 ·
Here are some pictures. You can see the silver tape that i put over the hole. One of the pictures you can barely see it.
 

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Now I`m thinking casting flaw, is this a perfectly round hole like it has been drilled..?
 

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I did a quick search master cylinder weep hole....they do have one and that is eactly what he has, but why is his booster leaking thru...must be a bad booster or seal around the master rim...learn something new every day..
 

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Some boosters had a flat washer type seal in the booster cavity where master cylinder goes in.The master push rod goes thru the center to seal booster and prevent vacumn being pulled on master cyl
 

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No not unless you got air in the hydralic system..
 
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