Random Brake Problem

'89 Cherokee, 4.0, aw4

Starting yesterday, at seemingly random times, my brake pedal has no resistance, and will drop to the floor... It still brakes just fine, it just takes a few more inches of brake pedal travel, and I really dont like not knowing when my brakes are going to kick in. Is this a sign my master cylinder is going out? Booster seems to be air tight (according to the test in my Haynes manual) so I do not believe it is that.

Thanks,
Jeff

I would start off with a MC. How many miles are on your rig?

It has 157k miles on it. Most of the brake system (except pads, shoes, and 1 hose) are original.

Classic master cylinder failure.

Bad booster would not cause a sinking pedal.

I replaced the MC yesterday and the problem is still there. I did bench bleed it, and everything is hooked up right. I checked again and still do not see brake fluid leaking from anywhere.

Any suggestions on where to look next?

Thanks
Jeff

Just replaced the extremely worn rear shoes, still no change in the odd pedal travel dilemma. Tomorrow I am going to bleed the entire system, maybe there's too much moisture in the old fluid in the lines, unless someone has some input .

Thanks,
Jeff

Well bleed the lines until there is all fresh fluid in the system. I do that close to once a year for all my vehicles, I've had original brakes on 230k mile vehicles that worked perfectly, only pad/shoe changes and new rotors.

Rear Wheel Cylinders are a common leaker, that could leak down the Reservour and soon as you suck air, 1 or the 2 stages goes limp. Plus if its a minor leak, it will usually self bleed itself, so if you add fluid, the pedal goes back to normal. Perhaps fluid sloshing as the vehicle moves uncovers and recovers.

Maybe a Rear Wheel Cylinder with an intermittent leak, it extends to a certain point and looses pressure.

Eitherway, you re-did the rear shoes, did you notice any fluid on the Wheel Cylinders?

How is the fluid level in the reservour as this happens? If your losing fluid, its a leak, fluid level stays the same, I'd guess a MC blown seal, but you just replaced that.

And last, brake leaks can be hard to spot sometimes, I looked for one on my Mini-Van for months, never spotted it until I had someone pump the brakes while I looked them over, then I noticed the fresh stream of fluid running down a caliper as the brakes were pumped. Same spot I looked over several times before, didn't notice it until I saw the fluid leaking down the side in action.

The rear wheel cylinders had no fluid on them... in fact, they still had some mud on them from a wheeling trip a few months ago.
When I replaced the MC, the original one was leaking past the seal, because as soon as I loosened the 2 nuts, fluid started coming from behind the piston.
I have been checking the level on the MC daily, and it is not going down at all, unless it's going REALLY slow.
I will look for leaks with someone pumping right before I bleed them... no sense in bleeding them if I have to make a repair and bleed them again is there?

Thanks for the help,
Jeff

Do you have ABS? Weren't there wierd problems and a lifetime recall or warranty on the Renix ABS systems?

You need to bleed the system better. Make sure that you follow the correct sequence. Start at the master on the truck with the line closest to the booster. Move to the front line after that. Then you want to go RR, LR, RF, LF and bleed each wheel until you get a clean clear stream of fluid for atleast 2 times. If there is resistance now but still a low pedal make sure your rear brakes are properly adjusted as this will give a low pedal. If that is fine check to see if the rod coming through the power booster is adjustable. If so, you may need to lengthen it a little bit at a time to get the correct pedal height due to production variances between the master cylinders.

Nope, no ABS.

I bled the lines at the master when I installed, and quite a few times too, but I will do it again before I start to bleed them after work. I have plenty of brake fluid . I adjusted the rears last night after I did the shoes, and a re-adjustment will probably have to happen, so I'll do that after I bleed the system.

speed bleeders will make it much easier to bleed I have speed bleeders all around and it is nice

Alrighty... Went to bleed system at the lines at the master cylinder and noticed my new one was leaking by the piston and pushing fluid out back by the booster, so I swapped it out again for another new one. New one bench bled, installed, bled lines at master, bled lines at wheels (RR,LR,RF,LF), checked for leaks on all brake lines along the whole lengths and at master cylinder again (making sure I didnt get another bad one), and then test drove.

The problem is still there! Went to the gas station on my test drive, and pulling up to the pump, brake pedal dropped to floor, and my E-Brake light came on. Went back off with another press of the brake.

I believe this is telling me (as well as the crud buildup in the bottom of the old master cylinder) that I have a sticking proportioning valve.

Anyone have experience with this? Input is appreciated.

Jeff

XJHummerRescue said:

Alrighty... Went to bleed system at the lines at the master cylinder and noticed my new one was leaking by the piston and pushing fluid out back by the booster, so I swapped it out again for another new one. New one bench bled, installed, bled lines at master, bled lines at wheels (RR,LR,RF,LF), checked for leaks on all brake lines along the whole lengths and at master cylinder again (making sure I didnt get another bad one), and then test drove.

The problem is still there! Went to the gas station on my test drive, and pulling up to the pump, brake pedal dropped to floor, and my E-Brake light came on. Went back off with another press of the brake.

I believe this is telling me (as well as the crud buildup in the bottom of the old master cylinder) that I have a sticking proportioning valve.

Anyone have experience with this? Input is appreciated.

Jeff

Click to expand...

it's not a hard or expensive fix, just replace it and know for sure

You may be correct on the Prop. valve. The symptoms you still describe would lead me to believe it is the MC, but I wouldn't be too hard pressed to say it may be the Prop. valve.

I'm a little skeptical, if the proportioning valve was leaking and losing fluid, then yea, I could see it being the cause. But you've said the fluid level is staying the same.

The proportioning valve/combination valve, does two things. It reduces pressure to the rear brakes and it has a piston/switch arrangement between the two stages of the brake system, so if there is a big pressure difference between the 2 stages it will move from the pressure difference and throw a switch to light the e-brake light.

The pressure reducer could block or do no reduction at all; eitherway the pedal would still feel the same and the rear brakes would be locking up or not braking at all.

The pressure difference switch, a properly working MC would have equal pressure on both stages, so no matter how blown seals were on the pressure difference switch the pressure would be the same on both sides of it.

That e-brake light is only going to come when there is a big pressure difference between the 2 brake stages. And the brake is only going to drop to the floor if there is a pressure drop. The only thing I can think that could cause boht;

*A Brake Leak (but your not losing fluid from the reservour)
*Blown Seals in Master Cylinder (your on your 3rd MC, that doesn't make sense either)
*Somewhere in the brake system, a huge expansion is being allowed without much resistance.

If your rear drum brakes were all messed up, they could be allowing one of the pistons in the wheel cylinders to move out really far and cause a pressure drop? Maybe a caliper has a piston being force back, so that it takes a full pedal stroke without building pressure? A hose is ballooning?

The rear brake hose is new, got one for a '95 YJ when I put in my lift, but the front ones are still original... so maybe one(or both) of the front ones could be ballooning. I really doubt with brand new shoes in the rear that either of the pistons could extend far enough to cause the pedal to drop like it does, but the piston in the caliper is plausible I think. Any way to check that?

Jeff

I've seen bent up rear shoes that let the pistons in the rear wheel cylinders extend way to far and cause a soft pedal. But, yea, brand new shoes, as long as the hardware and the pins and backing plate were in half decent condition, I can't imagine how the rear drums could still be the problem.

Hoses ballooning, enough to make the brake light go off and the pedal drop to the floor? I was probably a little presumptious in that suggestion, I'd think the hose would have to balloon so bad to do that, it would have had to burst long ago.

Caliper, try pushing the calipers in the direction of the piston, you shouldn't be able to push in that piston by hand, you need a clamp or lever, like a big screwdriver, to get it to retract. If something is driving that caliper back while your driving, the piston is pushed in, there forms a big gap between pads and caliper, then the next brake pedal push, no pressure until the pedal strokes enough to push the piston back out to take up the gap. Never heard of happening before, but that is the only thing I can think of that could cause you to loose brake pressure without loosing fluid, and is not a bad MC. Once the piston is fully extended again and the caliper all snugged up against the pads, the brake should act normally until the piston is forced back in again.

Again, a lot can be learned by having a helper pump the brake while you observe the brakes at each wheel.

The proportioning/combination valve, I'm far from some brake guru so take my advice with a grain of salt, but from how understand how that part works, I just can't see how it would cause the brake pedal to drop to floor or the brake light to go off, unless it was leaking.

Well, I did check the front hoses, and they are definately not ballooning. I went over every inch of brake line again, and there are no visible leaks (brake fluid would definately be visible on the light brown mud covering almost every inch of the underbody and almost everything under the hood ) I will check the calipers, but I dont see how anything could force the piston back in.. but I have to check, im almost out of options here :explosion. What symptoms would the brakes show if the vacuum line to the booster had a leak in it? Same as a wearing/failed booster, or different? I would just disconnect it and try, but it's pretty stuck on there and I dont want to make a leak if I can help it .

Jeff

Edit: Forgot to mention, I am noticing that EVERY SINGLE time I pull into my parking spot at work, the pedal drops, as well as my driveway. It seems like it happens more at low speed and rpm. I dont know if this could be related or coincidental, but it may be important.

A vacuum leak to the brake booster would give you less brake boost, the pedal would be harder to push down.

The Brake Booster only helps you push the brake pedal, if its bad, even in a way to make it too easy to push the pedal to the floor, then a good brake system would react just like you had slammed the pedal to the floor, it would lock up all the brakes. I got the impression from posts that the brakes don't change when the pedal drops to the floor, that can only be a pressure drop in the hydrualics of the brakes and definitely if you getting a brake light.