XJ - towing and brakes

[Warlock1]

What is the status of your proportioning valve, none, stock XJ, WJ, KJ?

You definitely have a problem in your rear braking system, gotta figure out what it is.

Does the new rear disk brake system have separate, drum in the rotor style parking brake?

 

[Warlock1]

Thing is, I've checked that the rear calipers are retracting the correct distance from the rotor, and they're moving at the same rate as the front.

What exactly do you mean by "retract"? Disk brake caliper pistons don't actually retract. When the pressure is released (foot comes off of pedal) the slight amount of runout of the rotor is what slightly pushes the piston back in the bore and allows the rotor to turn. If you are seeing a space develop between the brake pads and the rotor, that doesn't sound right.

I am not sure about the KJ calipers, but some other calipers, like GM with parking brake, have a self adjusting feature.

 

[8Mud]

1. Checked lines for air tonnes of times. I DOUBT I'm missing anything here.
2. No kinked lines as far as I can tell upon visual inspection.
3. Same deal.
4. Flex line to the rear is pretty old and could be a problem. But assuming it is, wouldn't it just slow the fluid to the rear, so the longer I hold on the brake, the better the braking would get? I can hold it down with all my weight while pushing my seat till I break a mounting bolt (ask me how I know), and the rears still won't stop the engine.
5. bleeders are all at the top . I've even pulled them while bleeding to make sure the bleeder was deadly at the top.

I don't really know if he has a problem or not. Most of the proportioning valves are designed to keep the rears from locking up.

Maybe one of the big name parts places has a brake pressure gauge to lend. Or http://www.amazon.com/SSBC-A1704-Brake-Pressure-Gauge/dp/B003VYVFSS

Most master cylinders are rated by diameter, the larger the diameter the more the volume, but less pressure (with the same applied force).

Braking power actually has less to do with pressure than it does with the brakes ability to generate heat. The more heat it generates the better it brakes, it changes mechanical energy into heat energy. May be he is running out of volume, may be his pressure is low, may be his pads are too hard or the disc is too smooth (glazed). Low volume or low pressure would show up on a pressure gauge.

 

[8Mud]

What exactly do you mean by "retract"? Disk brake caliper pistons don't actually retract. When the pressure is released (foot comes off of pedal) the slight amount of runout of the rotor is what slightly pushes the piston back in the bore and allows the rotor to turn. If you are seeing a space develop between the brake pads and the rotor, that doesn't sound right.

I am not sure about the KJ calipers, but some other calipers, like GM with parking brake, have a self adjusting feature.

I was thinking basically the same thing. Most times there is no air gap between the pads and the disc, just no pressure to speak of.

 

[md21722]

Trailers like the larger U-Hauls have brakes built into them. Even so, you could be pushed through an intersection while coming to a stop if the trailer is heavy. Putting it in 4WD helps. Better trailer brakes would be even better.

 

[donthelegend]

Sounds like everyone is already in agreement, but there is definitely something not right with your brakes. I run WJ brakes up front, stock drums in back on 33s and can lock the fronts up any time I want. Yeah braking distances pulling a trailer are longer, but I can still lock the fronts up on command.

 

[blistovmhz]

Sounds like everyone is already in agreement, but there is definitely something not right with your brakes. I run WJ brakes up front, stock drums in back on 33s and can lock the fronts up any time I want. Yeah braking distances pulling a trailer are longer, but I can still lock the fronts up on command.

Yea, my fronts will lock up when I'm unloaded for sure. If in 4x4, on dry pavement, the rears will lock up too, because the front brakes are then assisting rear braking. It's sorta sad that the fronts are strong enough to lock up front and rear, while the rear can't lock themselves up, nor overpower an idling engine.

Right now I've go the stock MC/Booster/PV installed. The KJ and WJ PV didn't have any effect whatsoever. Either way, the PV should only LIMIT my rear brakes on hard braking anyhow, which is the opposite of what I want. I did try without a PV at all, and same deal. Rear's just weak.

I might try swapping out the rear flex hose tonight if I can find a spare. If that's not the issue, I'm going to try to find a way to measure force directly from the calipers to get some hard numbers.

Am I the only one in the world to do a WJ front and KJ rear?

 

[lazyxj]

did you change the master cylinder to match the fluid volume needed for larger front calipers and rear calipers?
I'd guess that you will want something that moves more fluid to get the benefits of the brakes you have.

I run ZJ rear disc and Vanco 48-mm fronts (similar size calipers as the WJ) and have tried several different MC diameters.
https://www.flickr.com/photos/lwade/sets/72157616348032595/
The stock XJ MC provides the best performance, with enough volume and pedal travel. Bigger MCs may lack adequate boost to develop the pressure needed to stop the vehicle quickly.
My XJ's stock MC can lock up all fours 33"s, at freeway speeds, not that I want to.

Biggest problem I have is the increased weight transfer to the front, allowing the rear brakes to lock up more quickly, even using the ZJ proportioning valve.
Do wish it had a up to date ABS system for the wet and snow. I prefer our Subaru to slippery street conditions.

 

[blistovmhz]

I run ZJ rear disc and Vanco 48-mm fronts (similar size calipers as the WJ) and have tried several different MC diameters.
https://www.flickr.com/photos/lwade/sets/72157616348032595/
The stock XJ MC provides the best performance, with enough volume and pedal travel. Bigger MCs may lack adequate boost to develop the pressure needed to stop the vehicle quickly.
My XJ's stock MC can lock up all fours 33"s, at freeway speeds, not that I want to.

Biggest problem I have is the increased weight transfer to the front, allowing the rear brakes to lock up more quickly, even using the ZJ proportioning valve.
Do wish it had a up to date ABS system for the wet and snow. I prefer our Subaru to slippery street conditions.

Now I just gotta figure out if there's a difference in volume between the ZJ and KJ rear calipers

 

[lazyxj]

Now I just gotta figure out if there's a difference in volume between the ZJ and KJ rear calipers

Searching through Rock Auto's site, it appears that both should have a 48-mm bore.

 

[blistovmhz]

Searching through Rock Auto's site, it appears that both should have a 48-mm bore.

Someone correct me here if I'm making any incorrect assumptions:

Probably not:
* Rear brake line kinked or otherwise restricting flow.

* If this was the case, holding the brake pedal longer should cause the rear pressure to eventually increase to the same as the front, thus the rears would eventually receive full power.​

* Rears never receive full power.​

* MC rear piston is seizing before the end of it's stroke, thus rear pressure never builds high enough.

* This may not make sense because if the rear piston seizes, the front should also be stopped as there is brake fluid between the two. If the rear stops, so should the front.​

* Seal between MC front and rear pistons is bad.

* This probably doesn't make sense because if the seal was bad, pressure should equalize between the two pistons anyway.​

Possible:
* PV needs to be changed

* PV decreases pressure to the rear under hard braking conditions to prevent lockup. Perhaps the PV installed is simply too light and causing the rear pressure to decrease too early.​

*This could be tested and confirmed or ruled out by removing the spring and rubber cap in the PV, which should disable the PV entirely.​

* Seal between the reservoir and the MC rear piston is bad.

* This might cause the pressure in the rear to decrease, while fluid gets forced back into the reservoir? - Can anyone chime in on this one?​

*Only way to test would be to pressure test each line, or replace the MC.​

Probable:
* Rear pads/rotors not creating enough friction.

* I AM still using the old stock KJ rotors, but did grind it down nice and shiny and installed new pads.​

*Only reasonable test is to replace rotors.​

* Front WJ calipers require less fluid volume to provide the same clamping force as the rear KJ calipers.

* If this is true, the fronts would always have more breaking force than the rears, at all pedal travel.​

* Could confirm by checking rear brake line pressure against front brake line pressure, as they should differ because the MC front/rear pistons are ultimately going to be at the same position.​

So seems to me that the best course of action is to pressure test all four brake lines right? They should all be equal except under very hard braking, and under hard braking, the rear pressure should drop off. Knowing what pressure the drop off occurs, should tell me if the PV is too weak, and/or if the MC rear piston is leaking to the reservoir, and/or if the front caliper volume is just lower than the rear. Yea?

 

[yossarian19]

Well, you'd first need to know what pressure the PV is *supposed* to start working at. Without a reference, you'd at least be able to establish pressure reaching the rear, though.
My guess at this point is "back pads & rotors".
Interesting points about the MC operation, though.

 

[Panda_XJ]

Saw this a few months ago hopfully it helps. Upgrading to a larger bore Dodge Durango MC


https://www.youtube.com/watch?v=hMq_L59tZzw

 

[winterbeater]

Lots of theories here. One point I don't agree with though is the need for more volume for disc brake function. Because the pads don't move more than a couple thousandths of an inch, only a little fluid needs to move. The bigger reservoir is for when the pads wear and the caliper piston moves out to keep the pads against the rotor. Brake shoe movement on drums requires more fluid flow but less pressure.

Are you sure the power booster is working right? Try pressing the brakes hard with the motor off, then start the motor to check that the peddle goes down more.

 

[nospam865]

Just a thought, you mentioned your rear brake flex line was old. Could it be expanding due to age and losing pressure? Just my .02!

 

[87manche]

Lots of theories here. One point I don't agree with though is the need for more volume for disc brake function. Because the pads don't move more than a couple thousandths of an inch, only a little fluid needs to move.

You need more fluid to move it just a little bit, because the piston is larger.

 

[8Mud]

You need more fluid to move it just a little bit, because the piston is larger.

The down size to using a larger bore MC is you need more pedal force to generate the same force at the wheel.

Just a shot in the dark, air caught between the two pistons in the MC. I've had a few MC that were just a pain to bleed, nothing seemed to work. One I had to completely submerge in brake fluid to get it to bleed. There was likely another way to get it done, but I was loosing patience.

Then there was the rebuilt MC I got assembled with one piston in bassackwards, it took me hours to figure that one out.

Generally speaking the smaller the bore of MC the more pressure you get, with the same applied pedal force. The larger the bore of the wheel piston, the more applied force you get to the pads or shoes. I may be wrong about this, correct me if I am, it has been 50 years since I had this in school (I'm harvesting this from some really old brain cells ).

 

[blistovmhz]

Yea, regarding the MC bore, it SHOULD be more than enough, as my pedal is pretty firm and I only have to push it down about 2.5-3" before I just can't push any farther. If the bore was too small for my calipers, the pedal should go to the floor.
Now it could still be that ONE bore is too small (the rear), but the only way to really know is to pressure test everything anyhow.

I think the plan is going to be to build or track down a hydraulic pressure gauge and get that out of the way first. When I do it, I'll test with and without the PV, which should tell me almost all there is to know about the state of everything.

I did consider the rear flex hose expanding, but it's stainless braid and it ain't goona expand enough to cause light braking.

The other thing I thought of, is that when guys build bigger brakes for muscle cars, they generally either install a heavier spring in the PV, or just get an adjustable PV. Disc brakes do in fact, require higher pressure than drums, because they're not self energizing. I think it's entirely possible the PV is blowing out waaay too early, which would make a lot of sense as it seems like no matter if I'm an inch of brake pedal, or 3, the engine doesn't change RPM at all.

 

[8Mud]

The flexible brake hoses have been known to separate (internally) and cause issues. Though the issues are usually with the pads or shoes retracting (relaxing). I really can't remember any causing application pressure (volume) issues unless they were on the verge of blowing, had a blister or were really expanding. But if you run out of all the common explanations, it is time to start looking at the exotic.

When I sand the shoes, pads, drums or discs I use 100 grit coated sand paper (coated sand paper fouls less and can be cleaned). And sand in a cross hatch pattern like you would for honing a motor cylinder bore. This is the way I was taught and the way I've always done it. The guy who taught me was a pioneer in high performance (modern) brakes and likely forgot more than I'll ever know about brakes. He was also big on breaking in new brakes, taking it easy for the first couple of hundred miles and not overheating them.

Good luck, the pressure gauge is likely to tell you where your problem lies. I have some vague memory of removing the spring from the PV as a test. But like I said, it has been many years since I did brakes in any serious fashion. My specialty was the plumbing and I just helped out the practical engineers and system designers during installation on custom jobs. We did some neat stuff on street rods and race cars. A lot of brake guys died of lung cancer back in the 60's I decided the smart thing to do was to change professions.

 

[Warlock1]

Well, by process of elimination:

-You have verified that it is not the PV by not only removing the innards, but by swaping in 2 other ones. (which, by the way, you will need when this is all done and over with).

-You said that all lines are fine and free of kinks and dents.(did you verify that there are no blockages by debris in the lines that go to the rear brakes by blowing them out with an air compressor?)

-And that the calipers are fine.

Which brings us to the conclusion that it is probably a bad master cylinder. I would also do a vacuum test on the booster while you are at it, it's now 16 years old.

Oh, and by the way, the PV regulates the rear line pressure at all times, not just in hard stops. Anyone that has had to drive on snow or ice and baby the brakes would know this. The regulation is proportional (variable)to the amount of input.