When I bought my 70 SS, the brake warning light would come on intermittently. In order to pass safety inspection, I checked the fluid in the master cylinder and found the rear section almost empty. I refilled, bled the brakes, and all was fine. I was able to lock up the brakes if I stomped on the pedal, so I thought all was OK. One thing which seemed strange was that the cap of the master cylinder had blue silicone all around it, as if someone tried to make a gasket. I didn't understand why at the time, so I just left it alone.
Well, now I think I know why. After driving the car for a few weeks, I found a puddle of brake fluid under/behind the drivers side front tire. There are "streaks or runs" of dried fluid on the inner fender well, beneath the master cylinder, but I cannot tell if it is new or old. The face of the booster shows signs of pitting/rust and apparently has been leaking, but I've seen exactly the same signs on cars at swap meets, so didn't think that was the culprit. Any ideas as to the cause of the leak? I checked the area around the bleed valve on the wheel, but it is dry. How can I test this? Should the cap of the master cylinder have a gasket? Is it under pressure? It is a safety issue, as the car will not readily stop now. I can always refill and re-bleed, but I'd prefer to fix it and do this once.

Sounds like the cap was damaged/bent at one point in time and silicon was used to fix the uneven spots. Maybe if you tried a new cap to get a complete and flush closure to the cylinder. Sounds like just a quick fix deal. I have never seen a gaskey on a master cylinder.

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D Smith
ALABAMA
1970 Chevelle 350

If you want safety replace both rear wheel cylinders for sure. Master cylinder MAY be optional but I think I would just replace it while I was doing the wheel cylinders and be done with it. Might as well replace the rear brake shoes as well.About a hundred bucks to keep alive, pretty cheap.

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Harley
69 461 El Camino Nitrous Model, 69 Chevelle coupe
70 El Camino,71 SS Camaro
75 L-82 Corvette,71 Gremlin X
Kerrville,Tx.

Isn't there supposed to be a bladder type gasket on the master cyl. lid? Black thing that covers the whole inside of the lid. This prevents leaks while letting the fluid level go down over time. Junkyard item. tom

Thanx for the insight. There IS a bladder type gasket on the lid. Also, regarding the suggestion to replace the rear cylinders, there was no fluid leaking there - there was a puddle under the FRONT drivers side wheel, under the master cylinder. I'll try to replace the lid, as I have another master cylinder, but I was hoping to find the source of the leak first.

Its the bladder gasket,, I had the same problem on a 70 a few years ago,, bought a new gasket and it still leaked,, very confused I tried everything,, finally bought new master cylinder,, just before putting it on I thought I would try the cap I got with the new one on the old one,,,,shazammmmm,, dont know why it worked but it did,, sent new master back,,, L89SEDAN

Berry, if fluid is running doen the front of the brake booster it is most likely comming fron the back of the m/c where the rod goes into the m/c. I had that problem and when I replaced the m/c I found the fluid was also going into the booster. Do yourself a favor and get a new m/c. because the fluid will destroy the brake booster over time.

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BILL TIRELLI TEAM CHEVELLE MEMBER#280 1969 BLK CHEVELLE SS396 375HP 4SP 410 POSI
STATEN ISLAND NY

I went through 3 rebuilt master cylinders on my 70 SS -- they all leaked and were eating away paint on my freshly done frame. The third time, the car was about to go into storage, and I knew it would happen again, so I took a rag folded several times over and tied it around the back of the MC to catch any leaking out the back of the MC. When I got the car back in the spring, the rag had worked great, and after I took it off I never found another damn leak. I wonder if the gasket swelled over the winter to take up some of the slack between the cover and the gasket. I think most of the leakage problems are from the stamped steel MC cap -- they must not sit very flat, because I found brake fluid between the cap and the gasket -- which means that the gasket must not have been sealing tightly against the cap. Anyone know of a source for NOS master caps?