Dreaded lid leak on new master cylinder...ideas?

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gdizzle

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66 dart, recently swapped into disc up front, drum in the back.
Foolishly I assumed that the brand new dual well master cylinder would be leak proof, at least at the lid.

I got it installed and finally got it bled after 10 times, and now I see leaking coming from the top, and is nicely rusting the metal. The bail keeper is pretty tight already, I have to use a screwdriver to pry it on/off. The rubber gasket seems to not be able to fully seat/seal and getting lots of leakage mainly from the front of the lid. Has anyone figured out a fix for this?? Other than removing it again and filing down the mating surface? I prefer to not remove it (ever again). help
look at this horrible pic. this is 3 days old.
I just took it for a spin, and then looked at it again and fresh fluid was all over the upper fron portion of the MC, running down from the lid. All fittings are leak free.
mcseal.jpg
 
I am interested in this too, mine leaks and will have to repaint fender well. The company I bought it from basically said tough sh$t, recommend put RTV on it and they never called me back. Brake fluid is some nasty stuff.
 
You can always completely flush your brake system and replace the fluid with brand new DOT 5 fluid. DOT 5 is silicone based and non-hygroscopic (will not absorb moisture from the air). DOT 5 is also non-caustic and much gentler to your paint finish should you spill any - but you should still clean it from any painted surface quickly.
Read about the different types of brake fluid: Classic Car Brake Fluid
 
I went through that as well. I took mine off and used a belt sander on the top of the master cylinder to get a nice sealing surface. Cleaned, bled, put it back on and don't fill it to the top. That worked for me. Most are not flat on the sealing surface.
 
Cut a ring of gasket paper to fit between cap and rubber,the extra thickness may help the seal.
 
Is the keeper tight in the front and back or just the back?
It may be tight enough to need a screwdriver to remove it, but only because its tight on the rear
Had the same problem on mine, took the keeper off and bent the front of it a bit, no more problem
Just a thought of something to try before you go to any major fixes
 
Seepage, Creepage, Weepage. I painted mine so to keep from rusting. Keep wiping and dont open unless you have to. It should seal up eventually.
 
Some don't weep, they leak. The cylinder is cast and a very porous metal. Shitty fit for sure. Take it off. Sand it down to a nice flat surface and it won't leak. Or buy one with screw on lids.
 
I have found that most all of them will leak to some extent out of the box. I put one on a 72 ford the other day and its the best sealing MC I have ever installed. Zero leaks and the bail removes easily without pry tools....just lucky. I have had luck swapping tops. I think some tops are made better than others.
 
66 dart, recently swapped into disc up front, drum in the back.
Foolishly I assumed that the brand new dual well master cylinder would be leak proof, at least at the lid.

I got it installed and finally got it bled after 10 times, and now I see leaking coming from the top, and is nicely rusting the metal. The bail keeper is pretty tight already, I have to use a screwdriver to pry it on/off. The rubber gasket seems to not be able to fully seat/seal and getting lots of leakage mainly from the front of the lid. Has anyone figured out a fix for this?? Other than removing it again and filing down the mating surface? I prefer to not remove it (ever again). help
look at this horrible pic. this is 3 days old.
I just took it for a spin, and then looked at it again and fresh fluid was all over the upper fron portion of the MC, running down from the lid. All fittings are leak free.View attachment 1715302834
I haven't looked at one in years, but it looks funny to me. I admit I could be wrong, so don't roast me too bad. Is the cap on backwards?
 
Do not fill it to the brim. If the mating surface are clean and true it should not leak. If you have a fountain on brake apply the master was not bled fully.
 
I haven't looked at one in years, but it looks funny to me. I admit I could be wrong, so don't roast me too bad. Is the cap on backwards?
Good catch, most masters have the larger resevoir to the front, and cap should be installed to match. Fits even though the shape is slightly different.
 
I did same on mine after it leaked and messed up brand new paint on firewall.

View attachment 1715303047
I've had those long aluminum ones on my late B-bodies, and similar one on my Jeep, never had a problem with them leaking. It's kind of a ***** to change the plastic reservoir over, if you have to, but otherwise no problems.
 
66 dart, recently swapped into disc up front, drum in the back.
Foolishly I assumed that the brand new dual well master cylinder would be leak proof, at least at the lid.

I got it installed and finally got it bled after 10 times, and now I see leaking coming from the top, and is nicely rusting the metal. The bail keeper is pretty tight already, I have to use a screwdriver to pry it on/off. The rubber gasket seems to not be able to fully seat/seal and getting lots of leakage mainly from the front of the lid. Has anyone figured out a fix for this?? Other than removing it again and filing down the mating surface? I prefer to not remove it (ever again). help
look at this horrible pic. this is 3 days old.
I just took it for a spin, and then looked at it again and fresh fluid was all over the upper fron portion of the MC, running down from the lid. All fittings are leak free.View attachment 1715302834


Take the cover off and put it on a flat surface to check if the cover is warped. I have seen this before. If you have access to one, try another cover and see if it leaks.
Hope this helps!
 
66 dart, recently swapped into disc up front, drum in the back......
QUOTE]

I had the same leak. Turns out the rubber gasket is just a bit smaller than it should be. It barely covers the sealing surface. It's just undersized enough that if not positioned correctly, the gasket won't seal the surface on one side or the other.

My solution was to cut off the two tabs that hold the gasket into the stamped steel top and more carefully position the rubber gasket onto the reservoir before positioning the top, and re-position the retaining bail. I was careful not to move the gasket as I did this, and it's been sealed ever since.
 
On a flat surface, metal table top or sheet of steel, lay sandpaper down face up.

Take master off, flip over, rub over sandpaper like it's your job. When you think it's enough, do it some more.

Mine leaked day 1, day 2 until 11 years later not a drop.
 
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