SteveG75

Need some advice on a strange master cylinder leak.

Was taking out garbage this evening and noticed a puddle of what was later to be determined to be brake fluid by the front driver door of the Corvette (1975). A quick jack up and look with a flashlight traced the fluid path along the frame and under the master cylinder/booster area. Bottom of the master cylinder and booster both wet. Top of steering box wet. Popped master cylinder cover and both sides of master were down about 1/3 to 12 way.

So, cleaned up everything, used brake cleaner, refilled master. Made sure lines were tight. Loosened master from booster, no fluid came out of booster.

Brake test. Engine off. About 1-2" of pedal and then firms up. Pretty normal for my car. Engine running. 2" to 2-1/2" of pedal then firms up. Pedal never goes to floor no matter how hard I push.

I put a piece of cardboard under the cylinder and no evidence of leaks now. I did notice that when I popped the lid off to check levels that the the sealing surfaces on top the cylinder were all wet.

So, is it possible that the gasket and/or lid are the cause of the leak? This is the original master cylinder so it is 42 years old. Just not sure how the level could get so low with the car sitting still if that is the issue.

NAPA has new gaskets for $7 and master cylinders for $40 so a replacement would be easy (other than having to move my remote PS reservoir for clearance).

Any thoughts or ideas appreciated.

SteveG75

Just went and looked at car again. No fluid on cardboard yet but master cylinder looks wet all around the top by the gasket.

Is it even possible for brake fluid to leak out the top of the master cylinder when the car is sitting?

Zip has a gasket/cap/bails set for $26 and I get stuff from them in one day. So maybe I will try that first.

HeadsU.P.

First. Watch it on those lid cover gaskets. Some are too small, some are too large. No uniformity on that. Compare your new gasket on the MC instead of on the lid.

Second. New MC for $40? I bet its not the correct one. Try Corvette Central for photos of the many MCs available for sale. You do not want universal. Before ordering a new MC, take a look at the rear where the piston / seal is. There are two types: shallow and deeper inside the casting. Otherwise your pushrod will not mate correctly.

Not sure why you have B. fluid on the steering box several inches away. Maybe you hit the brakes hard when you parked it, IDK.

DUB

NO.....IF the brake fluid level is below the top edge where the gasket seals. Brake fluid can not leak out of the top.

It CAN leak out when you apply the brakes IF the seal is bad or the flat surface of the master cylinder itself is all screwed up and rough due to rust and crud....thus not providing a good seal.

For brake fluid to get on top of your steering gear box....it ha do come for the master cylinder. Because there is really nothing else that can get the fluid to that area.

DUB

SteveG75

Yeah, I would check for the right pushrod hole. NAPA lists different masters for the different years and power vs manual brakes. You would be surprised how much of the "Vette tax" can be avoided at NAPA.

SteveG75

Yup, undertand all that. I may just get a new master anyway. It lost a lot of fluid which makes no sense since I was just lightly tapping brakes (checking new bulbs) the other day. Very strange behavior.

Good thing is that I am not driving car right now. Bad news is that we are hopefully moving in a month or so and I need to get the car moved 900 miles.

HeadsU.P.

I understand that. But the pictures I saw of their units, well, just didn't look right. I would stick with a Corvette supplier to make sure everything matches up correctly.
And even then, some suppliers got it wrong.
Nothing against NAPA, I was there today. But you should be very fussy when it comes to brakes.

HeadsU.P.

Now we are getting somewhere. "I was just lightly tapping the brakes". Light tap, medium tap, hard tap, its still over 400 lbs of pressure in the system. So, it didn't leak just sitting there. If the MC IS going bad that would explain the leaking lid.

DUB

QUESTION:

When you initially removed the master cylinder cap and noticed the fluid level was down. Was the accordion seal inside the cap showing that is was also dropping down like it SHOULD do IF you were that low on brake fluid like you mentioned??? OR...was it still coiled up flat inside the cap?

I can say that if the surface of your rubber seal that goes in the cap is really crappy and NOT smooth rubber. I would more than likely replace the seal in the cap. The take a razor blade and a carefully clean the flat surface of the master cylinder so it will seal.

I could not nor would not sell you a master cylinder until I tried getting the cap seal to work first. Mainly because as you mentioned...the actual hydraulic function of the master cylinder is still correct.

DUB

SteveG75

Well, Zip has master cylinders for $80(replacement) or $129 (exact reproduction). I can also get a lid/gasket/bail set for $29. May be the way to go.

HeadsU.P.

Unless the lid is rusty, keep it. Unless the bails no longer hold, keep it. Just need a correct gasket. If you do go with a new MC it should come with lid, gasket, bail.

SteveG75

Just went out to garage. Cleaned up lid and reservoir real good. Lid is pretty rusty. Stepped on brakes a few times. No clear evidence of leaks. Time to run engine again.

HeadsU.P.

There. All fixed. That will be $195. How would you like to make your payment?

SteveG75

So, no obvious leaks but when I pulled up gasket from inside of lid, there was fluid between the lid and the gasket. Lid is pretty nasty too. Sealing surface on the master is not bad. I cleaned it up with a razor blade and scotchbrite pad (after covering the fluid wells) and it came out pretty nice.

So, going to order the lid kit from Zip and if that does not fix it the problem, I will replace the master cylinder next week (taking the week off work anyway).

Here are some pics of the lid (after I cleaned up some of the fluid inside it).

HeadsU.P.

E-w-w--w-! Thats really bad. That master was holding moisture for some reason. I see a complete flush of the system in the near future. Get yourself a bleeding vacuum pump tool. But meanwhile, start soaking the bleeder screws with PB Blaster.

SteveG75

Fluid has always been kind of murky. Sounds like a good flush is in order again.

I already have a Motive Products Power Bleeder.Car also has Wilwood D-8 calipers so not worried about the bleeder screws. Only issue with the D-8's is you have to unbolt and rotate the rear calipers when you bleed them due to the bleeder positioning. Done it before, can do it again.

calwldlife

Wow
i have not seen anything like that!
Clear why it leaks!
Good luck

HeadsU.P.

H-m-m-m. Never heard of that. Sounds like poor design or almost as if the calipers were on the wrong side of the car. How do you rotate the caliper when its attached with a steel line on the rears? Wouldn't you introduce air?

SteveG75

So, cap and gasket set or new master? Also starting to think about new booster since I don't seem to get much additional brake travel with engine running. Just not excited about trying to get to those nuts under the dash.

SteveG75

Wilwood D-8 does away with the steel line on the trailing arms for a flex line. It is a known issue with bleeder location on these calipers. Wilwood calipers are not side specific. They have 4 bleed valves at all 4 corners and the inlet is in the middle of the back of the caliper.