Master cylinder leak

J-man · Apr 9, 2012

I've tried everything to get my mc to stop leaking out of the top. I've tried 3 different caps as same result. Someone said to pull it off and bench bleed again. Does this sound correct or should I just buy a new one?

Apogee · Apr 9, 2012

Different MC (re)manufacturers use different materials for the bales that hold down the cap, some of which are complete junk and don't exert enough force on the rubber seal to actually seal. Certainly any issues with the seal and/or the sealing surface won't help matters, so start there and make sure there is no corrosion, pitting, etc on the sealing surface. If it looks good, then inspect the gasket for any flaws or issues. You've already cycled through enough caps to more or less rule those out, leaving only the bale(s). I don't know of anywhere to just get the bale other than used, but I've seen plenty of folks with a hose clamp or two around the MC to keep it from leaking.

Myllo73 · Apr 9, 2012

It may not be the master cylinder but another part of the system like the brake cylinders not moving correctly and backing up the pressure. Could have someone watch the m/c while the brakes are pushed to see if that's when it's leaking out. If not then it's worth the try to bench bleed before spending the money on a new one.

gtnorga · Apr 9, 2012

X2 on the hose clamps.
When I first got mine it leaked out of the top. The MC and fenderwell were wet. Two Big hose clamps accross the humps....my MC is bone dry now.

MarsChariot · Apr 9, 2012

Leaks out of the top are really easy to fix. Take the top off. Remove the rubber liner. Now get some really thin foam rubber weatherstrip material. Cut a length that will completely encircle the lip of the top where it seals agaianst the master cylinder. Trim the weatherstrip until it is a thin 1'4 inch or probably closer to 1/8 inch strip. Now place it around the circumference. Re-install the rubber liner. And use the original top clamps. It will not leak unless you have a major irregularity in the top of the master.

Pa PITT · Apr 9, 2012

I had one that leaked .. I did not think of putting more bales on it But I took mine off and took it too a very flat bench, I put down a sheet of Emmery paper and rubbed the M/C back and fourth across it to see if it was flat . I wanted it upside down so as to not get grit inside and cause more issues. I wanted to see if the top was flat. It still leaked but maybe not as back .. Later I needed a new cyl. and it didn't leak.

xcntrk · Apr 10, 2012

I'm with Myllo73; something is backing up the system causing it to blow out the weakest link (cap and seal). Either air in the system or locked up wheel cylinder. You can of course check the decking with a framing square or flat ruler to ensure the re-manufactured MC body has a flat deck just to rule that out. But I would search for whatever is wrong with your brakes causing the MC to leak. Also you can bench bleed right on the rig no need to pull the MC. Just pull the lines, cap the holes and bench bleed.

J-man · Apr 11, 2012

Thanks everybody for the advice.

Greg_B · Apr 11, 2012

Several years ago I installed a reman MC and it leaked like crazy... I tried clamps, sanding... sealant... all sorts of things... I took it back. The next one did exactly the same thing so I thought I was not bleeding it correctly... So I learned how to bench bleed it... same story... leaked like crazy.

The third one I installed only dripped.... I tried to make it work doing all the same things... bench bleeding sanding... large hose clamps... I got so frustrated I took it to a friend of mine who does fleet repair work and builds race engines for a living... I asked him what he thought. He told me to stop buying remanufactured MC's and get a new one. I had a hard time finding a new one but finally got Autozone to order one. Installed it the same way and have been running it for 7 or 8 years with no leaks. May start today but so far so good.

So.. I think there is an issue with some of the remanufactured units in our application....

Greg

bknbronco · Apr 11, 2012

Its common from what i have read. I just got one from autozone (i think it may have a lifetime warrenty), and its a verry nice one, a bit larger capacity and seals great, and best of all came with bench bleeding hoses and was cheep!

My first brand new one from a bronco vendor leaked around the pushrod and destroyed my brand new firewall insulator after just a day in the truck and it never left the garage!!!!

Dont settle for a fix on a brake master!! A few drops of that brake fluid can ruin a few hundred dollars of other parts!!

ol gray mare · Apr 11, 2012

If it's leaking from under the cap, go to the junkyard and get a cap from one that's not leaking. That fixed mine.

Master cylinder leak

J-man

Sr. Member

Apogee

Bronco Guru

Myllo73

Full Member

gtnorga

Full Member

MarsChariot

Planetary Offroader

Pa PITT

Bronco Guru

xcntrk

Bronco Guru

J-man

Sr. Member

Greg_B

Bronco Guru

bknbronco

Bronco Guru

ol gray mare

Sr. Member

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