If you know of a thread answering this question, please post a link as I can't find it.

So I've had this problem of my rear brakes not doing jack since I bought my Jeep in Nov. I've tried adjusting them, put new drums on (were shot anyway), the shoes are practically new, everything inside the drum is working fine.

I put the rear end up on jack stands so I could test the brakes with out using the front end. I can pull my park brake and I can lock the rear wheels up where the engine can't really brake them loose, but if I use the foot brake, the engine doesn't have too much trouble turning the wheels. The brakes are capable of locking the wheels as shown with the parking brake, but it seems for some reason I'm not getting the pressure I need with the hydraulics to really use the rear brakes.

I don't think there's air in the line as the peddle is nice and firm. The brake light doesn't come on either. Driving it you can't really tell the rear isn't working unless you're on a slick (snow or dirt) surface and slam on the brakes going slow enough to keep the ABS from kicking on. The front wheels lock right up, the back just keep rolling, even in the snow. Oh, and the wheel cylinders are fine too.

I'm stumped. Has anyone had this problem? The service brakes should be stronger than the park brake, so I know these should be working way better.

 

I thought that the rear did the least amount of braking anyways.

 

check your proportioning valve and make sure it's not leaking or faulty. Also, remember the rear brakes provide pedal strength and only 20% of stopping power, while the front provides 80%. if your pedal it stiff and it stops properly without fishtailing don't worry about it.

 

How do you test a proportioning valve? I want the rear to work properly, it's a safety issue to not have them working 100%.

 

What indication has the rear brakes given you that makes you think they are not working properly? ABS is not meant to "lock up" it's meant to help keep control of the vehicle. If the front are locking up you need to have the abs checked for codes. The abs proportioning valve is near the master cylinder and has a reset switch on it. it's under the electrical connector. unplug it and us a small screwdriver to press it in while someone pushes the brake. plug it back in and test drive.

 

I think what he is saying is that if your prop valve isnt leaking fluid and your e brake works then your rear brakes are most likely fine. If they were locking up then i would be concerned.

 

If I'm going 5 mph in snow/ice and slam on the brakes (not fast enough for ABS to kick in), the front lock up, the rear do nothing except keep rolling. In that scenario all four wheels should lock up because of the slick road. The ABS works fine at speed. Not having rear brakes is dangerous especially in winter, I don't think I need to explain that further.

Right now the parking brake is stronger than the hydraulic service brakes, that should be the other way around. The parking brake is just that, to keep the vehicle from moving when parked, the service brakes are designed to stop the vehicle.

 

I know what your talking about, mine do the same thing. I put mine in drive on the ice or snow with the brake on and the rears start spinning. I think I just need adjustment though. Sorry couldnt help you but I know what your talking about.

 

ABS activates (usually above 10 mph) in an unobstructed parking lot and apply the brakes firmly. It is easier to activate the ABS on a wet and slippery road surface. The antilock system should prevent the wheels from skidding. Pulsation may be felt in the brake pedal and you may hear a clicking sound. Avoid pumping the brake, even if the pedal is pulsating.

This is why you are having issues. the only way to road test abs systems is over 10mph. you say it works fine at speed. WHAT SPEED? if the brakes are working at speeds over 10mph they are working fine under 10mph.

The reason 80% of a vehicles stopping power is in the front, is so when you are breaking and steering at the same time you don't throw yourself into a violent flip or fishtail. Let me reiterate. If they work at over 10mph, they work under 10mph. 80% front-20% rear, less braking power in rear than in front. therefore more force is applied to the front when stopping. Ever notice that you only have to change rear drum shoes every other front brake job?

 

The ABS works FINE. Let me say that another way; I go fast (like 10 mph and up) and slam on the brakes and the ABS kicks in. The only way to really test the rear when driving is to drive slow so that the ABS does NOT kick in and thus allowing you to intentionally lock up the tires. That's what I'm doing. I'm driving slow and trying to get all four wheels to lock up. The front lock up just fine. The rear keep rolling. If I can't get my rear tires to lock up when I'm doing 5 mph on an icy road, SOMETHING IS WRONG. It doesn't matter that they don't do 50% of the braking. They should still lock up at 5 mph on ice. The front has to be more powerful because of the weight transfer when stopping, but that doesn't mean 100% of the braking is done by the front, I still need my rear brakes pulling their 20-25%, which they currently are NOT.

Saying the rear brakes work fine when the ABS kicks in at speed doesn't make any sense. If the front wants to lock up before the rear even engages, the ABS will kick in to keep the front from locking up even though the rear brakes aren't working, even more so because the front brakes are trying to do 100% of the braking and will lock up easier/faster. Oh, and just to clarify so we're all on the same page, I'm not worried about the ABS nor trying to diagnose the ABS. What I'm trying to diagnose is why I can't get my rear brakes to, well, brake.

As I've said a couple times now, I use the parking brake and I can get the rear to lock up no problem. When I use the foot brake, the brake that is supposed to stop the vehicle when I'm driving it, I don't get jack squat.

Sorry for the sarcasm, I just get frustrated when I have to repeat myself a bunch of times.

 

Does sound like you need to adjust yours since you're not getting any brakes at all. I at least get a little braking action, but its very very weak. They're about as close to useless as they can probably get, with out being COMPLETELY useless.

 

I did read an article on cleaning the proportioning valve wish I could find it again.

 

I also would suspect the master cylinder(bleed it,& see if you get pressure at both ports)

 

Long story short, I'm thinking I have a problem with one of two things via process of elimination.

A) Master cylinder
B) Proportioning valve

Basically there's nothing left to be bad on the rear end. I now have all new lines running back from the ABS, new shoes, drums, and wheel cylinders. Got it back together tonight and same story; parking brake remains stronger than the service brakes.

Does anyone know of any methods to test the master cylinder or proportioning valve?

 

adjut the rear brakes and be done with it