1978 with power brakes. Car is stock.Brakes were done in 2017 (previous owner, don’t know parts source) with SS lined rebuilt calipers (all 4), rebuilt master cylinder, new hoses, and new pads. The brakes have been dry and worked great until this September. Noticed that rolling up to a stop sign the car pulled gently to the right. Pulled the front wheels to inspect and found the driver’s front caliper has a weeping piston which wet the rotor surface explaining the pull to the other side (no grip vs grip). I knew the drill so rebuilt both front calipers and replaced the hoses and pads. Cleaned everything up and test drove. Brakes were back to normal.

After a 7 mile test drive I found the master cylinder cover had leaked enough fluid out of the back to run down the booster, drip onto the frame, and to the floor. Just one drop on the floor which happened right after the ride and no other leaking overnight.

I cleaned the fluid’s gravity path up. Removed the cover with no spitting or pressure detected. Checked the levels of the reservoirs and they were 1/4-3/8” below the top edge of the master. This master cover never leaked before. The bails for the cover seemed to be tight and I made sure the cylinder’s top was flat and clean then checked the cover for flat and clean. So, I replaced the old master cover gasket with a replacement from a Vette vendor. Same style. Another test drive and the same leak out of the back.

The old and new gaskets were the same thin style of rubber so decided to change to a different make. Got a Dorman gasket which seems thicker and has a heavier style of bellows. Lowered the fluid in the reservoirs to ~1/3 full for a test. Cleaned everything up. Installed the cover with the Dorman gasket and made sure the cover was on tight. Same 7 mile test drive on a cold car that has been sitting in a 55 degree shop for at least two weeks. Its was also 55 degrees outside today. Came back. Same little leak out of the back of the reservoir.

I just adjusted the rear bail to be tighter by putting it in a vice and hammering the center down a little. It is tighter now. Ran out of time so haven’t test driven it yet.

I’ve never had a master cover leak on a car. Searched the forum and internet for similar threads and info but only found suggestions of the same things I’ve already done (lower fluid levels, different gasket, check flatness, tighten bails). Nothing I hadn’t already done. This master cover was dry for years and since the brake job work it isn’t. What’s the problem and solution?

 

Fluid backed up into master cylinder when you compressed the caliper pistons causing it to overflow the master cylinder. No need to over think it.

 

I'm thinking that tightening up that bail should do it. That and keep the fluid levels 1/2 inch from the top.

 

When the caliper was re-installed and the pads were inserted would be the only time the pistons would have been compressed, no? The caliper hose wasn't hooked up to the hard line yet at that point so no where to back up to except the catch pan on the floor. Plus the levels in the master reservoirs were very low and the cover was off for the before and after the Motive bleeder was on the master. Maybe I'm misunderstanding your statement?

 

I'm hoping that's all it will turn out to be. I will go 1/2" on the levels & tighten the front bail too. Wish I had the chance to take it for a test drive to check it.

Oh, I followed some of your frustration with the high speed vibration. I had a similar vibration I chased for two years. It was around 65-70mph that I'd get a vibration. I would stay below the vibration speed or speed up through it and stay above. Balancing the tires helped reduce the vibration but it was still there.. For other reasons, I replaced the ~10,000 mile 2017 production BF Goodrich Radial T/A tires last month and the vibration is now gone. The car has been indoor stored the entire time those 2017 tires were on the car. I think BF Goodrich really had more problems with those tires than just the brown letters issue. The new tires are July 2023 production BFG Radial T/A and same size but they are smooth and quiet. No 65-70mph vibration anymore.

 

I had the same thing happen when I put a chrome cover on , I put the original back on and no more leak