Wheel cylinder, leaks but doesn't?

[wrightz28]

K, here's an odd one, and being formerly ASE certified in brakes has me at a loss.

Bare in mind, wife's been driving the jeep (98 limited) 24/7 now, so, not sure how much of her story collaberates........

Last night as we get in the truck to go to the store, she informs me that when she went to leave work earlier in the day, she applied the brake to put the trans in gear and the pedal went nearly to the floor and the brake light was on. She checked the resevior like a good girl and found it almost empty. Gimped it to the gas station and filled it up, applied the pedal and saw no leaks.

She drove home in near rush hour Chicago traffic, drove her friend home, then took our son to the mall, so I'd guestimate put at least 20-30 miles in total since this occourance. So, I pop the hood and check the resivoir which is still full. Galnce underneath, no visable leaks, until I sleuth around the backside of the tires and find the "missing" fluid on the right rear tire. So I crawl my happy little self under the truck and see that there is fluid on the drum and feed line is dry, thus assume leaking cap on the wheel cylinder. I have her apply the brake pedal numerous times with various pedal applications, soft and smooth application, and then slam it to the floor to duplicate the leak from the cylinder. No dice, nothing leaks, resivoir remains full.

So, curious to know, are any of these models prone to warping the backing plates? I had a Camaro quite a few years back that did the same thing, and replaced the wheel cylinders jsut in time to get me to work, only to have them "fail" again, later to to come to find that the plate warped and the pistons were pushing past the cup limits and leaking fluid that way.

Other than that, my only assumption is a out of round drum allowing the pistons to overextend just the same.

Ideas welcome.

[TRINI55]

two things to look at, if brake pads in worn out fluid level will drop, if master is leaking between cylinder and booster fluid could be sucking through vacuum hose and going through the engine, oil mark on rear tires could be axle seal leaking, remove wheel and check for symptoms mentioned, then lastly check for master cylinder leak,

cheers !!!!!!!!

[wrightz28]

Thanks, the leak is clearly from the wheel cylinder, just the one time and no where else.

[fredjacksonsan]

I'd check the level in the differential...if it needs a refill then keep an eye on the leak. I'm thinking it's an axle seal.

[Airjer_]

Theory one. Temperature, once the rear brakes warmed up all the pieces/parts expanded.

Theory 2. Enough cleaner fluid worked its way through the wheel cylinder cleaning the bore and seals enough to reseal itself.

[mastertech1964]

that is an odd one what about brake adjustment? or a combination of brake adjustment and an oversize drum allowing it to extend too far? maybe the adjuster finally did its job after an overtension of the cups in the wheel cylinder?

[wrightz28]

Originally Posted by Airjer_

Theory 2. Enough cleaner fluid worked its way through the wheel cylinder cleaning the bore and seals enough to reseal itself.

This is exactly something I thought of as it seems to be fine now, it hasn't lost anything and I've had her check the resivoir and basckside of the tire everytime she gets in and out of the truck. The rarity of it is, after near 20 years, I've never actually seen it happen.

Thanks for the axle suggestions guys, however, not sure how that got mixed in, but it was brake fluid on the tire, nothing else.

[wrightz28]

Originally Posted by mastertech1964

that is an odd one what about brake adjustment? or a combination of brake adjustment and an oversize drum allowing it to extend too far? maybe the adjuster finally did its job after an overtension of the cups in the wheel cylinder?

That is possible as well.

[wrightz28]

So, I replace the brake line to the rear end after overloading the truck and stressing it. WAS going to replace the wheel cylinders as part of a 'complete' overhaul of the rear end, but those lines were siezed up really bad. Misses wrightz28 wanted to go shopping, so problem for another day. After priming the system up and test driving, then inspecting for leaks. Nothing, nada, zero, zip, zilch leaking from the wheel cylinder? It's the craziest thing I've ever seen. And I had a 68 AMC with drum on all 4 corners. Nearly every other vehicle I've owned had rear drums and was ASE certified in brakes, and this just has me stumped.