97 TJ rear drum brakes won't overpower engine

[Lt_Salty]

Hey guys fishing for ideas here. New Member, first post.
Got my first jeep this winter and noticed the rear brakes didn't seem to be working. In the snow i would have the press the brakes very hard to stop and a few times it would lock the front tires up, but the engine with still pushing the rears, had to put it in N to stop.

side note : the front brakes were also pretty bad but they tightened up very nicely once i did pads and rotors, they work great now.

I got the rear in the air and put it in drive, i pressed the brakes and had to press them to the floor and the drums still kept spinning slowly or would just barely be able to over power the engine. (the torque converter does seem very tight) So i popped off the drums (they slid right off) and found pretty much brand new hardware, almost no brake dust on anything.
I tried adjusting the brakes with no change.
I replaced the drums and scuffed up the shoes cause they seemed slightly glazed. no change.
bled the rear brakes, no change.
Replaced the wheel cylinders and re-bled, no change
The driver side is adjusted so far out i can barely shimmy the drum on and off. the passenger side slides off easily but for whatever reason stops better than the driver side. I check the self adjuster and its contacting the star. i tried driving around the yard for a few minutes in reverse, hitting the brakes, but it doesn't seem to have changed anything. It looks like all the brake lines were replaced somewhat recently.

We are stumped. I know there's a proportioning valve under the master cylinder. Could that not be sending enough pressure to the back? I've read on here that all that does is delay the application of the front brakes?

any help/tips would be appreciated. thanks in advance.

 

[Jerry Bransford]

Are you in 4Lo when the brakes won't stop the Jeep? The brake proportion valve's job is to reduce brake line pressure to the rear brakes to prevent them from locking up which is what causes skids. The braking system is designed to provide something like 75% of the braking power to the front brakes. You definitely never want the rear brakes to lock up during a stop no matter how hard you stomp on the brake pedal.

 

[toximus]

It's normal for the rears to not lock up like the fronts when in 2wd. They should not spin however if you're at a stop with the brakes engaged with the rear on ice.

 

[mrblaine]

Hey guys fishing for ideas here. New Member, first post.
Got my first jeep this winter and noticed the rear brakes didn't seem to be working. In the snow i would have the press the brakes very hard to stop and a few times it would lock the front tires up, but the engine with still pushing the rears, had to put it in N to stop.

side note : the front brakes were also pretty bad but they tightened up very nicely once i did pads and rotors, they work great now.

I got the rear in the air and put it in drive, i pressed the brakes and had to press them to the floor and the drums still kept spinning slowly or would just barely be able to over power the engine. (the torque converter does seem very tight) So i popped off the drums (they slid right off) and found pretty much brand new hardware, almost no brake dust on anything.
I tried adjusting the brakes with no change.
I replaced the drums and scuffed up the shoes cause they seemed slightly glazed. no change.
bled the rear brakes, no change.
Replaced the wheel cylinders and re-bled, no change
The driver side is adjusted so far out i can barely shimmy the drum on and off. the passenger side slides off easily but for whatever reason stops better than the driver side. I check the self adjuster and its contacting the star. i tried driving around the yard for a few minutes in reverse, hitting the brakes, but it doesn't seem to have changed anything. It looks like all the brake lines were replaced somewhat recently.

We are stumped. I know there's a proportioning valve under the master cylinder. Could that not be sending enough pressure to the back? I've read on here that all that does is delay the application of the front brakes?

any help/tips would be appreciated. thanks in advance.

Pretty common. If you can get the drums off by hand without backing the adjusters off, they aren't adjusted correctly. Adjust them at the drum until you feel dragging but can still turn the tires. Once that is done, go do some stops or severe slowing with the hand brake until you feel the hand brake loosen up a bit. Take it back and readjust the drum shoes until they drag. Dig around and find pics of the self adjuster issues and make sure that is correct.

That should improve things. If you want or think you want more, the proportioning can be adjusted. Post up if you want to know how after you do the other.

 

[John Scoggins]

T

he brake proportion valve's job is to reduce brake line pressure to the rear brakes to prevent them from locking up which is what causes skids

The proportioning valve part of the combination valve allows normal fluid flow during 'moderate' braking, it only control fluid flow during 'high deceleration' stops. Once the brake fluid is not flowing, the pressure is balanced (at whatever the pressures are between the front and rear brakes as controlled by the pressure differential valve within the combination valve) throughout a properly functioning brake system. So the rear brakes should hold the vehicle at idle if operating correctly.

 

[John Scoggins]

i pressed the brakes and had to press them to the floor

Does the brake pedal still go to the floor after all the work done? If it does and the rear drums are adjusted correctly you might have an internal leak in the master cylinder on the secondary piston and/or a bad or plugged combination valve.

 

[mrblaine]

T

The proportioning valve part of the combination valve allows normal fluid flow during 'moderate' braking, it only control fluid flow during 'high deceleration' stops. Once the brake fluid is not flowing, the pressure is balanced (at whatever the pressures are between the front and rear brakes as controlled by the pressure differential valve within the combination valve) throughout a properly functioning brake system. So the rear brakes should hold the vehicle at idle if operating correctly.

On the TJ, the proportioning function is just that, proportional and the rear circuit rises at a pressure rate directly proportional to the front up to the knee point. If the pressure stops building and is held at any point, the rear does not change relative to its proportional level of the front pressure. What does change is the pressure after the knee point on the pressure graph. At about 75% of full system pressure, the rear pressure rise slope flattens out and doesn't rise as fast as the front.

I had to build a test bench with a TJ master and combination valve with 4 gauges on it to get my head around all of it.

 

[astjp2]

On the TJ, the proportioning function is just that, proportional and the rear circuit rises at a pressure rate directly proportional to the front up to the knee point. If the pressure stops building and is held at any point, the rear does not change relative to its proportional level of the front pressure. What does change is the pressure after the knee point on the pressure graph. At about 75% of full system pressure, the rear pressure rise slope flattens out and doesn't rise as fast as the front.

I had to build a test bench with a TJ master and combination valve with 4 gauges on it to get my head around all of it.

What kind of pressures are on the system Blaine?

 

[Lt_Salty]

Are you in 4Lo when the brakes won't stop the Jeep? The brake proportion valve's job is to reduce brake line pressure to the rear brakes to prevent them from locking up which is what causes skids. The braking system is designed to provide something like 75% of the braking power to the front brakes. You definitely never want the rear brakes to lock up during a stop no matter how hard you stomp on the brake pedal.

Braking in the snow when i had the issue was in 2wd high, it obviously isn't an issue in 4wd hi or lo since the frot can't lock up without the backs doing so aswell.

Pretty common. If you can get the drums off by hand without backing the adjusters off, they aren't adjusted correctly. Adjust them at the drum until you feel dragging but can still turn the tires. Once that is done, go do some stops or severe slowing with the hand brake until you feel the hand brake loosen up a bit. Take it back and readjust the drum shoes until they drag. Dig around and find pics of the self adjuster issues and make sure that is correct.

That should improve things. If you want or think you want more, the proportioning can be adjusted. Post up if you want to know how after you do the other.

I did see the post about the correct configuration of the self adjuster cable, even though it seems like its touching the star correctly, ill take it back apart and run it the way its shown in the guys picture. After that I'll keep adjusting them out. The wheels are already pretty hard to turn in N (I guess because I have a Limited slip rear?) so dragging any more is gonna make it very hard to turn. but ill give it a try.

When you guys put new shoes on, how far out did you guys have your adjusters? if i remember correctly, i think my adjuster is out almost half an inch.

 

[Lt_Salty]

Does the brake pedal still go to the floor after all the work done? If it does and the rear drums are adjusted correctly you might have an internal leak in the master cylinder on the secondary piston and/or a bad or plugged combination valve.

no brakes are pretty firm i did originally have a leak when i didn't tighten the brake line enough when i installed the new wheel cylinder, but i have checked a few times and its not leaking anymore, at least not at the drums

 

[mrblaine]

What kind of pressures are on the system Blaine?

We see from 1100 to 1700 psi with the stock booster and master with the pressure gauge at the end of the hardline on the frame. My test bench will go up to 3000 psi.

 

[John Scoggins]

On the TJ, the proportioning function is just that, proportional and the rear circuit rises at a pressure rate directly proportional to the front up to the knee point. If the pressure stops building and is held at any point, the rear does not change relative to its proportional level of the front pressure. What does change is the pressure after the knee point on the pressure graph. At about 75% of full system pressure, the rear pressure rise slope flattens out and doesn't rise as fast as the front.

Yes, we are saying the same thing as far as results, the proportion valve controls the pressure rise by controlling flow rate delivered to the front and rear brakes. The manual states that moderate braking the rate is equal, at hard braking it slows the flow to the rear brakes via a smaller restriction than the front. The manual then states that the pressure differential valve controls the respective pressure, that valve is in the same block so that block is called a combination valve.

 

[mrblaine]

Yes, we are saying the same thing as far as results, the proportion valve controls the pressure rise by controlling flow rate delivered to the front and rear brakes. The manual states that moderate braking the rate is equal, at hard braking it slows the flow to the rear brakes via a smaller restriction than the front. The manual then states that the pressure differential valve controls the respective pressure, that valve is in the same block so that block is called a combination valve.

Yes and no. There is no control or proportioning to the front fluid circuit. It comes out of the master and enters the combo block with commonality between the 3 ports in non ABS systems. Essentially the back of the combo block is nothing more than a Tee for the two front brake lines with no control or restrictions on or to them.

The rear pressure is the one that is proportioned. The front is always the pressure at the port in the master.

 

[John Scoggins]

Yes and no. There is no control or proportioning to the front fluid circuit. It comes out of the master and enters the combo block with commonality between the 3 ports in non ABS systems. Essentially the back of the combo block is nothing more than a Tee for the two front brake lines with no control or restrictions on or to them.

The rear pressure is the one that is proportioned. The front is always the pressure at the port in the master.

Got ya,

 

[JoshNZ]

Reviving this thread rather than starting a new one.

The other day I classified myself as someone who "knows" a lot about cars in theory, but I'm still working on the in-practice part.

The above all makes sense (to me) in theory, so now for in practice

I replaced my rear shoes last weekend, adjusted them to what I think is correct, and upon testing, found the Jeep stops straight and consistently. I can lock up the fronts on greasy roads if I try, parking brake does what it's supposed to (now)

However, with the rear on stands, with the trans in drive, I can't stop the rear wheels with the pedal.
With 4.56 gears which I assume doesn't help.

Can I conclude the above to say this is OK?

 

[mrblaine]

Reviving this thread rather than starting a new one.

The other day I classified myself as someone who "knows" a lot about cars in theory, but I'm still working on the in-practice part.

The above all makes sense (to me) in theory, so now for in practice

I replaced my rear shoes last weekend, adjusted them to what I think is correct, and upon testing, found the Jeep stops straight and consistently. I can lock up the fronts on greasy roads if I try, parking brake does what it's supposed to (now)

However, with the rear on stands, with the trans in drive, I can't stop the rear wheels with the pedal.
With 4.56 gears which I assume doesn't help.

Can I conclude the above to say this is OK?

Not really. All the ones that I consider to be working correctly will stop the tires at speed on jack stands.

 

[Goatman]

So, did I read that the rear brakes work fine in 4hi? But not 2hi? Post #9. Never heard of the brake system changing its operating system depending on transfer lever.

 

[mrblaine]

So, did I read that the rear brakes work fine in 4hi? But not 2hi? Post #9. Never heard of the brake system changing its operating system depending on transfer lever.

You haven't heard of it because you didn't apply any working knowledge of the t-case to the problem. The t-case doesn't have a differential in it so both drive shafts are essentially connected together with no differentiation between the front and rear. That means the front brakes are now stopping the rear tires and vice versa.

 

[JoshNZ]

Not really. All the ones that I consider to be working correctly will stop the tires at speed on jack stands.

Thanks for the clarification. I'll get to work

 

[Lt_Salty]

You haven't heard of it because you didn't apply any working knowledge of the t-case to the problem. The t-case doesn't have a differential in it so both drive shafts are essentially connected together with no differentiation between the front and rear. That means the front brakes are now stopping the rear tires and vice versa.

Yeah what he said. My front brakes were braking all 4 wheels through the driveline.