Master Cyl Leak

95 XJ 133k mi. Brakes began to feel mushy. Checked master cyl, fluid at add level. I noticed booster was damp where master cyl mounts. A week or so later fluid was low. I figure master cyl leaking and replaced with new unit. Same problem. Booster is wet and fluid needs added. After 100 hiway miles fluid level will go from full to add. Pedal is low but doesnt bleed down while holding brake at stop. Whats the prob? A dud new MC? Vaccum booster? Any suggestions? Thx, Bill.

Sounds like you need a new brake booster.

billw said:

After 100 hiway miles fluid level will go from full to add. Pedal is low but doesnt bleed down while holding brake at stop.

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Sounds like a leak, rear drums out of adjustment will give you low pedal height, but not half way.

One stage empty will have the pedal down more than halfway.

Is the moisture brake fluid, water or something else? Theres lots of space in the booster to trap leaked fluid, the new moisture could be left over from the old master cylinder and forced up from using the booster.

I've replaced a Master Cylinder, had leaking brake fluid in that area again and returned the master cylinder under warranty for another one, Only to find my leak was actually somewhere else in the brake system.

Did you look for leaks at the slave cylinder, lines or hoses in the rest of the system. Old Rear Brake Wheel Cylinders very commonly leak, you really can't see it unless you pull the rear drums and inspect.

The brake system is a dual system, there are 2 tandem pistons in the master cylinder, the rear piston creates hyd pressure for half the brakes and the hyd press also pushes the front piston forward to create pressure for the other half of the brakes. If you have a brake leak, it eventually leaks dry, and one of those pistons no longer has fluid in front of it, so it collaspes (thats the pedal height dropping so low) and once it hits the other piston or the end of the master cylinder it stops and that lets the other piston create the pressure for the other half of the brakes.

If the fronts were leaking, you'd lose the front brakes, Just the rear brakes would drastically reduce braking performance and be unstable, its very noticeable if you lose the front brakes. If the rears were leaking, it would just be the front brakes which does the overwhelming majority of the braking, performance and stability would not be that effected if you lost the rear brakes.

Thanks for replies. There are no visable leaks in the brake system other than the brake fluid on the booster. This is the troubling part, the leak at the master cyl mount on the booster is worse now than with the original master cyl. I have added 1qt of fluid in 2 weeks. Could a defective booster ,perhaps an internal vac leak cause this? In the past, any leaking master cyl I have had didnt hold brake pressure while holding brake at a stop. Anybody else ever have this problem? This is a new master cyl, not rebuilt...I am glad for any/all help. Thanks, Bill.

Does the fluid around the Brake Booster look like its accumulating at 1qt per 2 weeks? Thats a lot and everything in that corner of the engine compartment should be coated in brake fluid, if that much is leaking from the rear of Master Cylinder.

Could it being sucked into the motor thru a vacuum leak from the booster?

Have you pulled the rear drums and inspected the rear wheel cylinders. They will leak lots of fluid and it flings off the back of the drum and doesn't really leave much evidence behind. You have to pull the drum and then you can see all the leaking brake fluid.

Like I said, I've had a little bit of fluid behind a brand new Master Cylinder before and there was nothing wrong with it, it was a leak somewhere else.

Have someone pump the brakes while you look at each point for a potential leak. That how I found my one leak, I saw fresh fluid flowing down from the leak (it was the seal between the new brake hose I put on a caliper, I overtorqued it and crushed the gasket) that I had looked at before and never noticed the fluid collecting there.

I know its not a safe diagnostic proceedure, but out of desparation I pulled and plugged vac line to booster. Drove 3 days and no loss of fluid. I dont know whats inside the booster or how this could occur. I'll replace the booster and see what happens. Bill

A bad booster is not going to make your mc leak. However, if you have any brake fluid in the booster, it needs to be changed. The only way brake fluid gets into the booster is if the mc is leaking from the rear piston.

Kyung

I had a shop install a "new" mc a couple of years ago and when I got the Jeep back I kept noticing a trace of fluid down the back of the mc, onto the booster. I popped the cap off and discovered that the mc was made from a bad casting. Along the top, where it seals with the cap, there should be a nice flat machined surface, but at the rear in one corner there was not enough material to be machined, so it had a rough casting surface. Just enough to allow fluid to seep out. Just one more thing to check for.