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Resolved: Brake fluid leaks from top of master cylinder reservoir

Anybody had this problem before? Early 70’s Ford truck. The brake MC fluid is filled but not overfilled, and doesn’t leak at all when the truck is parked. It can sit there for a month and never leak a drop, parked. But when I drive the truck, even just around the block, then park, and look inside the engine compartment immediately the brake MC is dripping brake fluid. It appears the leaking mode is by the fluid somehow jumping up and coming over the top edge of the MC when the truck is in motion or when the brakes are applied. All my diy’er years of replacing brake master cylinders on Ford Galaxies, VW Rabbits, Corollas, and this truck, I’ve never seen this kind of weird brake problem before. On this MC design it seems there’s sort of a rubber baffle inside the lid of the MC, and that’s present and accounted for. Any ideas?


Remove the cover for the master cylinder.

And when you turn it over, do you see rubber pointing up?

If so, push the rubber back down so it looks like this.


Also be sure that it’s really leaking from the top, and isn’t leaking from the seals between the reservoir and the master cylinder. When those seals die, you get all the drips of a reservoir leak except for on the reservoir itself, and since the reservoir is often dirty and oily it can fool you into thinking the leak is coming from higher up than it is.

If it IS leaking from the reservoir top, then once that’s fixed clean those seals very well because the inner surface of those seals are impervious to brake fluid, but the outer surface often is not, which will allow the fluid to dissolve the seals from the outside in and cause a leak.

I’ll check that rubber cover again, but I’m pretty sure that’s the way it looks already @Tester. @shadowfax this particular MC design doesn’t use a separate reservoir, it’s all part of the same ass’y. Here’s what it looks like.

It would sure be nice if more of them were like that, @GeorgeSanJose – At a quick glance that’s a much better design than a plastic tank that relies on bendy rubber bits to make a decent seal.

Sounds like the brake fluid is sloshing out the top. Driving will slosh the fluid around, parking won’t.

It is very likely one of two things may have occurred. 1) the metal cap may be bent a bit so it doesn’t make a seal or 2) The gasket is just so old (45 years…) it won’t seal anymore.

Ok, some experimental results to report. I removed the rubber gadget from the lid, cleaned it and all the mating surfaces, both on the lid and on the MC rim, reinstalled with the rubber pointing up, and down, no difference, still leaked from the fluid sloshing around while driving.

The gasket isn’t 45 years old btw @Mustangman , truck isn’t quite that decrepit, lol , the master cylinder ass’y was replaced 3 years ago.

From everything I can see it is just fluid sloshing over the top of the MC, somehow getting by the gasket. Which seems nearly impossible. It looks to be mostly leaking at the front edge, the edge furthest from the firewall. Not entirely surprising since the MC is tilted slightly downward, maybe 2-3 degrees, so the fluid will be more likely to slosh over there. Also that front section is for the rear brakes I think so it has a smaller reservoir than the back part for the front brakes.

I tried pressing on the brake pedal numerous times and vigorously while the truck was stopped and engine off, did this before the drive test, and no signs of leaks occurring just from pressing on the brake pedal. Almost certainly from the truck’s motion.

I think the theory about the cap being bent is possible. It was probably a rebuilt unit that I purchased as a replacement. I’ll take a straight edge to the cap and the top surface of the MC, see if I can determine anything. It didn’t leak at first though. I think I still have the old cap from the failed MC from 3 years ago, so will try that too.

I did notice when cleaning the cap there’s two tiny holes at the top, but I think those are supposed to be there, to allow air in as the fluid level drops from brake use. In any event the leak isn’t coming from those holes b/c the rubber gasket is in the way and no signs of brake fluid getting into that area.

Also thinking of using some thin gasket material and making a gasket to exactly fit the rim of the MC. that would be a good opportunity to also test the theories in the other thread how to make flimsy gaskets stay put.

Try removing the bale wire from the master cylinder, and bend it so it pushes down on the cover harder.


I messed around w/the position of that wire, no joy, but good idea, I’ll try bending it downward in the middle so it presses harder.

Edit: Also considering installing a couple of big hose clamps that you tighten w/a screwdriver or nutdriver around the entire MC ass’y.

Maybe the flange on the master cylinder or the cover is warped. Maybe that could be checked with a small straightedge.
Unusual I admit but when dealing with automotive any possibility is open.

7.13.2016 Resolved

I removed the lid and bench-inspected it for flatness using a straight edge. Definitely bent a little, like boat, higher at the ends than in the middle. Maybe 1 mm out of flatness. I turned it upside and whacked the center part w/a hammer, which seemed to straighten it out pretty flat. I put in on a flat surface and it didn’t rock at all. But that only tests the edge overlaps, which aren’t the sealing surfaces. Measured the sealing surfaces with a straight edge, each edge measured straight. I had no way to test the sealing surfaces w/respect to each other tho. Reinstalled, it still leaked. Sigh.

So went w/my backup plan, look through my pile of old parts and find the other lid from the last time I replaced the master cylinder. I didn’t turn it in for a core refund b/c I was thinking of using it to build a pressure bleeding system. It took a while, but I finally found the old master cylinder & its lid. Comparing the lids, the geometry was the same, but the old one seemed to be made of heavier material, and it didn’t have any small vent holes in the top, like the current (leaking) one has. The gasket material seemed the same between the two, same type of rubber and same thickness, same dimensions.

What the heck, I installed the old one, took the truck for a test drive on a bumpy road, not a single leak! I’m not entirely sure why, but the old doesn’t leak at all.

Also that front section is for the rear brakes I think so it has a smaller reservoir than the back part for the front brakes.
I think you have that backwards. Disks require less fluid to activate than drums, and the chambers are divided according, IIRC.

Drums all around.

Well, it’s easy enough to follow the brake lines to see which reservoir goes to which brakes. I’d say the bigger reservoir would go to the front brakes because they’re more “important.”

I didn’t catch the “early 70s” part of the vehicle description. It’s been decades since I owned a car with drums all around. I believe my particular car ('71 Ford Maverick) had equal size reservoirs but this was not a truck, and it was a long time ago!

I’ve been thinking of fitting disc brakes on the front. Kits are available for around $500. The way it is now, drums-on-all-4, it stops ok, that’s not a problem. But for some reason it tends to pull one way or the other a little during hard stops. I can adjust it out using the brake adjuster wheels, but it comes right back in a few months. I’ve never had a brake pulling problem on cars I’ve had with disc brakes.

I remember we were discussing this a few years ago

And Rick was adamant that drum brakes were simpler and superior

I believe he also did a few hard braking tests of his own and concluded that discs were in fact superior. I’m not sure if his vehicle experienced pulling during braking, but he did experience SEVERE brake fade

I feel drum brakes might be acceptable for some large commercial vehicles, but not for the kind of vehicle that the average person drives to/from work

Drum brakes still have their place on passenger vehicles, but not on the front axle

The drum brakes on my Maverick stopped working almost completely if I went through a puddle. They don’t build 'em like they used to, and thank goodness.