[dataway]

Below are some photos of a sudden leak that has developed at the rear of the OEM MC on my 68 GTO.

Fluid has been in this MC for several years with no leaks. Previously the car stayed in a garage with very stable temps around 55 degrees. The last 18 months it's been in new building where the temps have varied between low 40's and 70's, and higher humidity. It is filled with DOT 3 fluid.

At first I thought it was from the rear seal of the MC, but from the photos I can see that it's coming from either the back of the cap, or maybe the the hole where the wire loom inserts. What I don't understand is the fluid will leak down to where the rear reservoir is almost empty (without driving the car), the front reservoir remains full.

Surely the capillary attraction of brake fluid is not so high that it climbs out of this reservoir and out the rear cap seal? The leak does appear to be coming from high in the back, right in the area of the MC cap seal tab that goes through the cap.

Cap goes on tight, zero leaks in the lines and fittings. I haven't open the MC in a couple of years at least, never leaked a drop until the last month or so, even though I'm not driving it.

Thanks,

BTW .. what is the hole for at the bottom of the MC? Mounting a proportioning valve?

[jwcfbd]

It's weird that it would just start leaking for no reason but things fail. Open it up and take a look at the rubber seal. It may have just broke down and started leaking.

[dataway]

I had the cover off, seal appeared fine, perhaps a bit swollen. What is strange that it still leaks when the level is way below the level of the cap.

I did notice that the expansion joints had moved completely down. I see no signs of leaking wheel cylinders (four wheel drum), pedal is still firm.

It's almost like it's cracked somewhere, but that seems unlikely.

Will probably have to pull it off, hate to do it, I spent a lot of time getting the system bleed and working great a few years ago when I installed SS lines.

[OG68]

I wouldn't be surprised if you found the rear seal leaking.

The Disc Brake hold back valve attaches to the firewall mounting studs

[Scarebird]

Originally Posted by dataway

...The leak does appear to be coming from high in the back, right in the area of the MC cap seal tab that goes through the cap...

That sounds like the culprit right there. I would examine the cap to see if it sits flat or is twisted.

[1969GTO]

On a different note: I also own a 68 GTO with a power brake [ drums] booster.
Is the color of your booster an original color?
I have to ask because mine is black and I'm wondering if for some reason in the cars life that someone painted the booster black.

[tjs72lemans]

I had two different problems like that at different times. The first was there was a crack or open porous spot that leaked fluid through he casting. The second was the rubber seal on the cap was not sealing even though it looked fine.

[mgarblik]

The hole in the back of the MC at the bottom is a weep hole, just like a water pump. When the seal fails it's supposed to drip between the booster and the MC out that hole and not fill up the booster with fluid. I would remove the MC and clean it up good and replace the internal seals with a piston/seal kit. Easy and inexpensive. If there is pitting or deep rust in the bore you need a new MC or that one could be sent out and sleeved with a bronze sleeve if you need to keep it numbers matching. If it has light rust, a quick clean-up with a hone will fix it right up. I would also recommend switching to DOT 4 fluid. It's 100% compatible with DOT 3 but absorbs less water from the atmosphere. Cost just pennies more per pint.

[gtospieg]

Where are you getting seal kits to rebuild a Pontiac mastercylinder?

[mgarblik]

Our sponsor Ames has them. Just got one for my 62 Cat. Haven't installed it yet. Never have the time to work on my own cars!!

[gtospieg]

Thanks, I'll look into that.

[dataway]

I rebuilt the MC not long ago (about five years ago, didn't leak after the rebuild), the leak is definitely coming from up high, cover seal looks fine, but the top of the casting isn't the smoothest in the world. I should have lapped on a surface plate before rebuilding it.

Good tip on the DOT4, I might do that considering it's compatible.

I've removed and messed with the cover seal several times now, sometimes it leaks afterward, sometimes not. It's a repop cover and seal of course. Seal seems reasonably firm, not like it's dissolved or anything.

The actual junction between the MC and the booster is dry.

69GTO, no it is not original. The original appeared to have a more silver/white/gray color , but then it was 50 years old when I replaced it so not sure what the original color really was.

[mgarblik]

It's possible the lid seal is the issue I guess. Fluid should never get on the top side of that seal. As you probably know, by design the metal lid is vented to atmosphere. As brakes are depressed, and a small amount of fluid it trapped in the MC, calipers, wheel cylinders, air enters under the metal lid and equalizes the pressure between the lid and the rubber seal. If fluid is low, the seal bellows will expand down and take up that space. This helps keep moisture out of the system. But if that seal has failed, some fluid could get on top of it between it and the lid and leak out. Also between the casting and the lid.

[Nobuddy]

DOT 4 actually absorbs more moisture than DOT 3 but it does have a higher boiling point when pure. So it may or may not be an upgrade depending on how often you change your brake fluid. Not a huge deal either way and many fluids on the shelf will be labeled DOT 3&4 in same bottle.

[mgarblik]

Originally Posted by Nobuddy

DOT 4 actually absorbs more moisture than DOT 3 but it does have a higher boiling point when pure. So it may or may not be an upgrade depending on how often you change your brake fluid. Not a huge deal either way and many fluids on the shelf will be labeled DOT 3&4 in same bottle.

That's interesting. I need to research that further. Almost all DOT4 fluid sold is labeled LMA. (For low moisture absorption) . I may be learning something new.

[dataway]

So I finally got to work on the MC. The sealing surface for the cover had some minor raised rust areas, I lapped the surface flat.

There was also some rust in the rear seal area which I should be able to remove with a light hone (always looks worse in pictures).

My question is, do you think the seal is still good? That pedal hasn't been pressed 20 times since that NOS kit was installed a few years ago. In the photo the upper piece is what was installed, the one below is my backup. I'm trying to save that seal since these US made kits to fit the original MC are getting VERY hard to find.

I was using DOT3, and I also read that DOT4 absorbs more moisture than DOT3. So I plan to stick with the DOT3. I'm thinking the poorly sealed cover might have let in moisture that then settled in the seal area of the MC bore.

Not sure what the three seals are, they were loose in the opened NOS kit I have as a spare.

Any comments welcome.
Thanks

[mgarblik]

The piston and seals on the second picture, (the one without the rust on it), looks better. The rusty piston, the rear seal has swollen some and doesn't look good to me. If you look at the pic, it has more room to move and "fold it's lip" to seal when the brakes are applied. The older seal looks to fill up the entire groove. Unless this is all an optical illusion. The remaining extra seals go on the other piston not in any of your pictures. There should be another piston in the MC along with a free spring that goes in the bore first.

[Shiny]

Now that you tore it apart, what do you think made it leak? Those seals look swollen but intact. You think fluid got by them where the bore was rusted/pitted?

Could you re-install them and somehow test them on the bench? Maybe bench bleed, then install some short pinched lines and lean on it?

[dataway]

It appears to me that in both instances (cover and seal) that rust lifted the sealing surface and allowed fluid to slip by. This would also jive with the way it just started leaking out of the blue with no use. Went years without leaking, then after sitting for a couple of years in our new humid TN location it just all of a sudden started leaking just sitting there.

Of note is that is rust in both locations where it is leaking.

[dataway]

mgarblik, those two pistons, although they should both be suitable for this application are different part numbers, not sure how much that is effecting the seal size.

The rear seal does look swollen a bit, not sure if that's typical after long term exposure to DOT3 or not. I haven't removed the front piston yet.

Shame that no one is making a kit for the all drum, power brake MC anymore.