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Brake Master Cylinder leaking

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doughairfield

doughairfield

Jedi Trainee
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The master brake cylinder is still leaking on my TR4 even after a rebuild on it's innards. From what I can tell, it's losing brake fluid out of the cap, but there shouldn't be any pressure under the cap right? I mean you bleed the brake system with the cap off. I did replace the rubber gasket under the cap but the cylinder has the original screw on cap, which I have tried to tighten the heck out of, which also doesn't help.

Has anyone else encountered this problem?
 
Hey, Doug -

I seem to remember that TR6Bill reported on a similar issue a while back - had something to do with improper cap/sealing issue.

Do a search of the Triumph section and TR6Bill and you should find it.
 
The cap has an air vent. You wouldn't happen to be overfilling the reservoir? When you brake, the fluid heats up and expands, if you overfill, the fluid will need some place to go. The cap is only really supposed to prevent the fluid from splashing out when you go over a bump.
 
If you over tighten a plastic cap they can distort, or crack and leak.
 
I've searched for TR6Bill and can't seem to find that post, but I did find a reference to it in his bleeding brakes thread.

I don't think I'm over filling it, my driveway is pretty level and I fill it right up to the line, not a drop more.

I've got the original metal cap but a new rubber gasket. But I was thinking I could have been over tightening, I have tried not tightening the cap to much to see if that was the problem, no luck.
 
I don't think getting the cap wicked tight is the answer + you risk damaging the hard-to-find original metal cap.

I'm trying to recall the line you refer to. I usualyy have mine filled to just where that angled flat bit on the front meets the round shape (geez, does that make any sense?).

I would try having less fluid in the reservoir. If you check it often (and don't we all) you don't need a huge amount in there.
 
I agree with Geo and his suggestion. Mine isn't tightened very tight at all. Here are how my cap/gasket and fill level appear.
BrakeMasterCyclinder001.jpg


BrakeMasterCyclinder003.jpg
 
Note in Jay's photos how the cap sticks out in the middle on both sides. That inner bump is a deflector of sorts, and can sometimes fall out (or be removed). Without it, you may get fluid coming out the vent hole, especially if you have a habit of pumping up the brakes.

Also check the top edge of the reservoir, to be sure it's flat and smooth.
 
Thanks for the suggestions, I'll check for that bump in the underside of the cap tonight, I don't recall seeing it last time I had the cap off. And I'll clean the rim of the cylinder off and make sure it's nice and smooth.

I fill my reservoir to that line around the inside of the cylinder, just like Jay's picture in his post so I feel like my fluid level is good.
 
Hopefully this pic is not too grainy.

It looks like that bump is present on both sides of my cap, so that part looks good. It looks like on closer inspection that the fluid is leaking out from under the cap on the front side of the master cylinder.

I cleaned the rim of the master cylinder up as much as possible but it looks like time as not been kind and there is some significant pitting on the rim and the threads and the threads are a bit worn. It also looks like there aren't any threads as they come around the front of the cylinder either.

I'm gonna try putting the cap on without screwing it down to tight and see how that goes if no one has any suggestions from the pic.
 

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The missing threads probably mean you are stuck sourcing another MC.

However, yours might be repairable. Drain & remove it from the car, then stuff a rag or paper towel tightly into the bottom (to keep the swarf away from the hole).

Then use a flat file to touch up the lip of the reservoir. Work the file at different angles, but always supported by both sides, and you should be able to get a clean, flat surface.

Now paint the top edge with machinist's blue or a black magic marker, and install the cap without the rubber seal. Look for the high spots where the cap hits the lip, then file those off (again trying to hold it as flat as possible). You won't get it perfect, but it shouldn't be too hard to get close enough that the seal can take up the difference.
 
:iagree:

And you might try using a belt sander with 120-150 grit belt. It will provide a flat surface. Put light, even pressure on the MC.
 
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