Leaky brake mastercylinder resevoir

[JGrefenstette]

since the summer my brake master cylinder resevoir has started to leak intensly from the top. it sloshes around and spills out under the rubber gasket. i mean i was filling it back up every other week because it would spill out over half the resevoir. the frame rail below was wet all the way to the driverside door. so i replaced the rubber gasket and the steel arm/strap that holds it down. its very tight on. hoping that i would pop the hood after my next drive and find that the resevoir would not be soaked totally, i poped the hood and find it is STILL leaking out the back 1/4 of the resevoir top! what else is there to do? i filll it up to about 1/4 to 1/2 inch from the top. anyone else had this issue? that brake fluid is nasty stuff, i dont want it eating up everything it dripps all over.

 

[PToombs]

Mine was leaking too. I'm cheap, so I just flipped the rubber gasket over and it sealed. I'd look at the sealing surface. Maybe it's pitted from rust. You might be able to take a fine emery and block sand the top surface of the MC. Make sure you put a rag or something in the MC, don't want much grit in there! :-laf

 

[rlyons]

Take off the wire that latches the top on and bend it so it fits tighter. Mine was a little loose and leaking until I bent it a little, been dry since.

 

[Dale Reed]

Leaking somewhere that is for sure because the front smaller container is losing brake fluid, the entire master cylinder is wet and rusty, and there is brake fluid collecting on the frame below. So I will first try a new master cylinder top gasket. Apparently NAPA has one pretty cheap.



But can't figure out why the container should leak so much? Normally when I step on the brakes is there increased pressure on the fluid or is there something wrong somewhere else in the brake system that is causing the brake fluid to try to get past the gasket?



Thanks for the help. Dale

 

[JHumphries]

If the front reservoir is losing fluid, I would check the brake lines for a crack or leak.
A small enough leak would only leak under pressure and not affect braking performance.

 

[Gene Heck]

Check your rear brake adjustment to make sure the shoes aren't too loose. If they are when you let off the brake and they return it will cause excess brake fluid to shoot up and hit the rubber seal. If they seal is bad or those wire bales, clips, retainers, or whatever you want to call them are too loose it can cause brake fluid to leak out of the cap. Another thing, if the cap or seal is loose enough to leak fluid out it is loose enough to draw moisture into the brake fluid since brake fluid is hydroscopic. A big no no.

 

[Dale Reed]

This morning I ordered a new rubber gasket from NAPA and it will be delivered tomorrow.



Later I dried everything off, cleaned up the sealing surfaces of the rubber gasket and the top of the reservoir. Got most of the rust off and after a short drive the surfaces were wet again but I think that is from brake fluid that is still between the gasket and the removable top of the master cylinder. I did not see any more fluid anywhere outside the master cylinder.



After I replace the old rubber gasket with the new one I will be watching for any leaks anywhere but I have already looked over the rest of the brake system and have not seen any.



I adjusted the rear brakes with a screw driver a couple thousand miles ago and once in a while I step on the brakes hard when backing up which tightens the rear brakes.



The brakes work fine and I will do what I can to figure out what is happening to the fluid but if I require some professional help I have a mechanic who got his Cummins just before I got mine so he has fixed mine from time to time and he could instal a new master cylinder or at least purge and bleed new fluid into the system.



Thanks everyone for your helpful comments. I will keep you posted.

 

[Dale Reed]

Gene, you are saying that the smaller front container supplies the rear brakes. Makes sense since the front disk brakes normally require more fluid than the rear brakes so should be supplied by the larger rear container.



But if you look in the Disk Brake section of the 1990 Service Manual (page 5-34) it says:

"As lining wear occurs, the fluid level in the front brake reservoir will decrease. This is a normal condition and only requires enough fluid...



Maybe it has to do with the definition of "front. "



Assuming that the implication of your post is correct and that the manual is incorrect, then my vigorously backing up and stomping on the brakes to reset the rear brakes may be the primary cause of the fluid in the front section of the Master Cylinder dropping fast?



Today I installed the new rubber seal on the Master Cylinder and noticed some drops of water between the old rubber seal and the bottom of the lid.



After driving a few miles I see only a very small weep under the new seal and I don't see any wetness below the Master Cylinder. If it continues to weep I will figure out how to increase the pressure on the rubber seal.



Thanks for the help everyone. Dale

 

[Gene Heck]

Dale,



I can't say about your vintage for sure but on mine and most others that I can recall the end of the master cylinder closest to the booster serves the front brakes and the end closest to the radiator serves the back brakes. Don't depend on backing up to adjust the brakes. The starwheels stick over time with brake dust etc. You have to pull the drums and check how easy the starwheel turns on each side. You can pull the adjuster cable to cause the ratchet to move up and engage the starwheel on the next tooth and it should move it out freely one "click". If it doesn't do this with the drum off it won't do it as you back up and apply the brakes either. Or if you hold out on the rachet and the starwheel should turn without binding. If the brakes aren't adjusted tight enough the wheel cylinder has to have more fluid to move the shoes out and when you let off the brake pedal the brake return springs have to move all the fluid back into the master cylinder until the shoes stop on the anchor pins. It can cause a little stream of fluid to shoot about straight up out of the resevoir and hit the bottom of the resevoir seal. I've seen this stream shoot up 2 or 3 inches and over the side with the cap off.



You can bend those wire bales a little to make them hold the lid on tighter as long as the top of the resevoir isn't rusted and pitted. A little moisture between the cap and seal won't really hurt anything as long as it doesn't leak into the fluid. Washing the engine etc. can cause stuff like that.



Gene

 

[Dale Reed]

Gene what you say makes sense to me. This morning I took my truck into the mechanic that has worked on it for twenty years explaining my problem. I will call him tomorrow and ask him to look carefully at my rear brakes. If they do not need relining then at least he can tighten them up. I already suggested that he install a new Master Cylinder whether it is leaking or not. So with new brake fluid and assuming he did not find anything else wrong with the brake system the brake system of my old truck should be good to go.



I was thinking of not renewing TDR again this year but it is times like these I realize that as long as I own my old Cummins the best place to get informative advise is here.



Thanks again Gene.

 

[Dale Reed]

Checked in on my truck today and like you said the rear brakes were leaking. But now I have new brakes, tomorrow a new Master Cylinder, and brand new brake fluid.

I will ask how the rear brakes could be leaking so much but I could not see the leak by laying on my back and looking for leaks with a bright flashlight.

 

[waltk]

Where were the brakes leaking from? If it was inside the drum, it might not have got past the two sealing lips that interlock with the backing plate. When was the last time you checked you break lines? You may want to change them, or at least inspect them if the opportunity come around before they cause another, very similar, problem.

 

[Dale Reed]

Have not talked to the mechanic yet to ask him for details but he has replaced some brake lines both rear and front. Pretty sure the rear leaks were inside the drums. Unfortunately for my pocket book my mechanic usually finds more problems than I can imagine so my old truck will run mighty fine until I bring it to him again.