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Brake master cylinder

Brake master cylinder
E

eatdrinkadam

Jeeper
Posts
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Location
Seal Beach, CA
Vehicle(s)
1985 Jeep CJ7 4 cyl.
Hello all. I have a 1985 CJ7 4 cyl. I seem to be having a chronic problem with my brake master cylinder overflow running dry. I already had replaced the master cylinder once due to this problem, but it still seems to persist. Every 2 weeks my brakes will go soft and I will need to refill the reservoir and bleed the brakes for air. I checked around the brake system and did not identify any leaks. I do notice brake fluid tends to over flow and leak around the lid of the reservoir. This happened on both the old and new cylinder.. Perhaps this might be a problem with this type of cylinder? Does anyone else have this problem and have any recommendations? Thanks A
 
Sounds like you need a new rubber seal around the lid of your Master Cylinder.

Out of curiousity, are either of the calipers, or wheel cylinders locked up.?

brian
 
I would like to this so.. but the rubber seal is new as is the master cylinder. I can tell that the fluid leaks because the outside of the cylinder is totally rusted and is damp with fluid.

The brakes work fine otherwise.. so the calipers dont appear to be locked up.
 
eatdrinkadam said:
I would like to this so.. but the rubber seal is new as is the master cylinder. I can tell that the fluid leaks because the outside of the cylinder is totally rusted and is damp with fluid.

The brakes work fine otherwise.. so the calipers dont appear to be locked up.
Click to expand...


It may be a dumb question, but the new seal and the new MC came TOGETHER right...Just making sure the seal is meant for that MC
 
Is the top of the master cylinder body smooth and machined? My son just rebuilt a Ford pickup, and put in a new master cylinder. The sealing surface was unmachined and rougher than a cob and kept leaking. My master cylinder has a machined surface. So he took some JB weld and coated the top to fill in the imperfections and found some thicker gasket material for the seal. So far no more leak.
 
I never though of JB weld to smooth it out. Good idea.:chug:
 
If the master cylinder is going dry, it's leaking somewhere. Inside the rear brake drums? Have you taken the drums off?

A wheel cylinder?

Or a leaking joint?

The key is - it's running dry, not splashing out.
 
Well I am just assuming that the source of the leak is in the master cylinder because of the signs of rust and break fluid on the outside. When the reservoir runs dry (because the fluid is being pushed out) air gets into the brake line when pumping the brakes with no fluid.

There very well could be a leak on the inside of the drum.. I haven't thought of that. This could explain why I haven't seen any visual signs of leakage anywhere other than the master cylinder.

I will have a look-see.

Thanks for the idea.
 
eatdrinkadam said:
Hello all. I have a 1985 CJ7 4 cyl. I seem to be having a chronic problem with my brake master cylinder overflow running dry. I already had replaced the master cylinder once due to this problem, but it still seems to persist. Every 2 weeks my brakes will go soft and I will need to refill the reservoir and bleed the brakes for air. I checked around the brake system and did not identify any leaks. I do notice brake fluid tends to over flow and leak around the lid of the reservoir. This happened on both the old and new cylinder.. Perhaps this might be a problem with this type of cylinder? Does anyone else have this problem and have any recommendations? Thanks A
Click to expand...

The very first thing I would have done is check around the brake system as you said you did in your very first post. That would include looking for fluid that has run down the inside of your tire/wheel/ or drum (or disc) backing plates. Next would have been the line connections at the proportioning valve and splitters. When you said there was no evidence of that, the ONLY other alternative is the MC seal. (unless you have PB and it could be leaking into the power booster)

Thats the reason for the focus on the MC seal. As well as the evidence of fluid on the MC..

I'd spray brake parts cleaner on everything to get clean surfaces and run brakes again. Then check again for fluids. I'd be surprised to see fluids leaking from line splitters and prop valve, but you never know. Most likely are the wheel cylinders. I consider them to be the weakest link in the chain.
 
Old post and I may be a little late, but.... Not a mechanic or expert but a long time ago on Fords I used to see the problem of the master cylinder rod shaft seal leaking brake fluid into the power booster. It will fill this up and then leak back out. The fluid will eventually eat up the booster rubber parts.

Anyone else ever see this?
 
huntmaster said:
Old post and I may be a little late, but.... Not a mechanic or expert but a long time ago on Fords I used to see the problem of the master cylinder rod shaft seal leaking brake fluid into the power booster. It will fill this up and then leak back out. The fluid will eventually eat up the booster rubber parts.

Anyone else ever see this?
Click to expand...

Yes, I actually suggested this in post #10, but didnt get any real response either way... And yeah, it creates a hell of a mess!!

I dont know what the resolution to this problem was....
 
I had a Cherokee that did this and it turned out to be a caliper leaking. You couldn't tell because when you drive it the brakes get hot and it burns the fluid off so when I checked they were dry. But I think if it's going dry you must have a leak somewhere in the system.
 
Sorry to jump on this old post but where can I order online a rubber gasket for the master cylinder for 1982 master cylinder. Mine is currently twice the size it should be due to some fluid getting in and the rubber expanding. Please point me to the page where I can order. i found one place Autozone but I needed to be trade or something.
Ken
 
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