My buddy's 72 has power brakes. The bottom part of the master cylinder casting has like this hole cast into it which baffles the daylights out of me as to why it is even there, but anyway, there's a vaccum leak you can hear. If you tale your fingers and plug both left and right side of this opening, it suckes your fingers in and stops the leak. Can anyone explain what has happened here? I don't think this ever existed before. Some pics here. In tyhe last pic you can almost see a crack or something, inside the hole?
Re: Vaccum leak from master cylinder? Ok, just to clarify, the leak is coming from the casting of the master cylinder from that horizontal porhole that you see in the picture. Two questions. 1. Is the casting of the master cylinder bad? How is it even possible that you can have a vaccum leak at that location? 2. If it is a crack in the master cylinder, why would the booster need to be replaced? So far there's one person that says the master cylinder is bad and one that says the brake booster is bad. I'll wait for a few more to chime in.
Re: Vaccum leak from master cylinder? There is a rubber seal, cup shaped, in the brake booster where the master cylinder mounts. It may be damaged. Unbolt the ms and take a look.
Re: Vaccum leak from master cylinder? I had a sneaking suspicion that this might be the case, but I was not sure there was a seal behind the M C. Either way, even if it is a bad seal, there's a crack in the M C it seems? it's still not even clear to me what this opening's purpose is in the casting. Baffling.
Re: Vaccum leak from master cylinder? The orifice is open to the rear of the master cylinder for two reasons, one to allow brake fluid a way out if it leaks past the primary piston...when that orifice closes up with rust and debris, you can get leaking fluid inside the booster where it does more damage. Two, to equalize atmospheric pressure behind the piston when it's depressed. Devon
Re: Vaccum leak from master cylinder? Isn't that hole a designed-in weep hole incase fluid starts leaking out the back of the MS? It may extend up through the casting to the back end of the piston where normally there should be no fluid....or vacuum unless something fails.
Re: Vaccum leak from master cylinder? For there to be a vacuum leak, I believe the booster must be leaking. So it'll need to be replaced or rebuilt. The fact that it's leaking out the weep hole of the master cylinder is probably just due to where the leak is. I'd change the booster and then see if the master is working fine. It could be that both parts are on their way out, but the booster is the first step.
Re: Vaccum leak from master cylinder? Ok so in other words if the booster didn't have a vaccum leak, there'd be no vaccum leak at the weep hole to begin with?
Re: Vaccum leak from master cylinder? Exactly. The Booster is a self enclosed unit that helps press against the master cylinder piston. You should be able to take the Master cylinder right off and still not have any vacuum leak.
Re: Vaccum leak from master cylinder? The way see (hear/fell actually ) what the others are trying to explain to is remove the Vacumm hose from the booster - pump the brakes several times (to relieve residual pressure ) you will then have no pressure other than while the piston moving back and forth from your leak - and will see that it is coming from the booster - I would replace both - you need the booster - re-manufactured /replacement masters are cheap - might as well do the job once rather than twice, if it turns out that it is cracked.
Re: Vaccum leak from master cylinder? Was going through a recent booster failure on Shari's GS for the second time in a few years. Each time it was a nicely plated unit that I had paid a lot for. This time as usual it was from the front of the booster. This time instead of buying another booster decided to look into the problem. It turned out to be the front seal in the booster directly behind the master cylinder that is easily replaced. The part number from the seal pulled from the booster was #5452549, This number was changed to a #5460935 by GM in 5/77 then changed again on 2/85 to #18005270 but discontinued on 10/94. I went to Midwest Remanufacturers LLC in Bedford Park, IL (http://www.pwrsteering.com/) who would only sell me one seal for $10 as they were running short of this seal. It is hard to believe the little bit of missing rubber made the booster practically useless. The new part restored the booster.
Re: Vaccum leak from master cylinder? I just fixed this same type of leak last in January. Easy to fix. First, to confirm that rubber seal is shot, unbolt the master from the booster; leave brake lines in place. No nead to open them. You should have sufficient play in the lines to move the master out of the way and get the seal. Mine was torn from lack of lube between the seal and the pushrod - let that be a lesson to everyone. Go to you local pick and pull yard and look for an A body or F body up through probably 1982 (any other body that uses the same type master would probably work too). I found a '77 Transcamaro to be perfect. Find one with a decent booster (not a rusty disaster; preferably with a master on it) and remove the master. Take the seal; pay for it if you really want to give up that $1.20. You'll find that later model seal (flat) will work fine instead the early model (cup). So - total outlay: $1.20 plus gas, plus time.
Re: Vaccum leak from master cylinder? I have the same vacuum leak on my 69 Riv. The front seal however does not look the same as in the picture posted earlier in this thread, but it has the same part number (NPC 13 5452549 ) . I have attached a few photos below of the seal : The seal inside the brake booster might be the one that was shown in the earlier post? Does anyone know where I can get a hold of just this seal? I am afraid we don't have the luxury of going to the scrapyard in Norway to find an American car :/ I also noticed what seems to be an o-ring groove on the master cylinder. Would it be sufficient to use an o-ring there instead of replacing the damaged seal? Regards, Kenneth
Re: Vaccum leak from master cylinder? The flat rubber seal would easily fit in an envelope. How do we mail something to you and what does that cost?
Re: Vaccum leak from master cylinder? Do you have one I can buy? Sending an envelope (weighing 11 ounces) with USPS costs about 11$ according to their website.
