Brake Prop Valve Necessary?

[drcomanche]

So the short story is, I'm switching my braking to a dual diaphragm booster to stop the monster tires. When I took it out, I saw that the front brake line just went to a t connector and then hard lined to each sides ss to caliper. The rear was a single adjustable prop valve(that was seized). Because this is how it's always been, I never thought much of it, but I've also never had ideal braking characteristics(it tends to sway back and forth under a heavier brake load). I'm not sure if that is due to the uneven distance the fluid has to travel for one side or if the lines hold an equal constant pressure. After researching some, I found that there's generally a proportioning valve that mets out brake pressure. My question is, is that necessary? Will I have better braking results by adding that back into the system? 

 

I researched on here and found a thread discussing internals and ideal setups, but that was all with relatively stock brakes. My Jeep runs on a d44 9 inch axle combo with the large disc brakes. I really don't know if that makes a difference. 

[schardein]

Since you have a custom set up, I'll say two things:

 

The stock prop valve (or more properly, combination valve) is nice because it incorporates a brake warning light if either the front or rear circuit develops a leak.

 

I would consider replacing your adjustable prop valve if it is seized.  I've done several disc brake conversions, but the first was on a 79 K5 Blazer with 3/4 ton axles, Chevy 14 bolt in the rear.  I used an adjustable prop valve AFTER the stock combination valve.  There was a coupling in the brake line half way back along the frame that was a perfect place to put it.  It was there so the same front line could be used on all the trucks, but different rear line depending on wheelbase (Blazer, long bed, short bed, suburban).  Most of the aftermarket adjustable prop valves can reduce braking pressure up to 66%.  I had to turn that adjustable prop valve almost ALL the way down to even out the braking effort.  That's how effective the new disc brake setup was.

 

By retaining the factory combination valve, I still had a warning light, with no detriment to braking ability.

 

Best place to test is a straight gravel road with a slight downward grade.  Get going about 15-20 mph and hit the brakes.  If the rear locks up first, it will try to slide around.  Be ready.  Then adjust the pressure to the rear down until the front locks up just before the rear, and the rear doesn't try to swing around.

[Eagle]

2 hours ago, schardein said:

Since you have a custom set up, I'll say two things:

 

The stock prop valve (or more properly, combination valve) is nice because it incorporates a brake warning light if either the front or rear circuit develops a leak.

 

I would consider replacing your adjustable prop valve if it is seized.  I've done several disc brake conversions, but the first was on a 79 K5 Blazer with 3/4 ton axles, Chevy 14 bolt in the rear.  I used an adjustable prop valve AFTER the stock combination valve.  There was a coupling in the brake line half way back along the frame that was a perfect place to put it.  It was there so the same front line could be used on all the trucks, but different rear line depending on wheelbase (Blazer, long bed, short bed, suburban).  Most of the aftermarket adjustable prop valves can reduce braking pressure up to 66%.  I had to turn that adjustable prop valve almost ALL the way down to even out the braking effort.  That's how effective the new disc brake setup was.

 

By retaining the factory combination valve, I still had a warning light, with no detriment to braking ability.

 

Best place to test is a straight gravel road with a slight downward grade.  Get going about 15-20 mph and hit the brakes.  If the rear locks up first, it will try to slide around.  Be ready.  Then adjust the pressure to the rear down until the front locks up just before the rear, and the rear doesn't try to swing around.

 

In the MJ, that block up front (which it would appear drcomanche doesn't have) is NOT a proportioning valve. It offers NO proportioning function whatsoever -- it's a distribution block, plus the device that triggers the brake failure warning light on the dashboard.

 

Pickups especially need some sort of proportioning valve because when the bed is empty it's VERY easy for the rear brakes to lock up before the fronts, causing a spinout. But -- when the bed is fully loaded, you need all the braking you can get on the rear. This is why the XJ has the proportioning function built into the front distribution block, but the MJ uses a rear proportioning valve that responds to how heavily the bed is loaded by increasing the braking to the rear wheels under load.

 

The first question is: does the truck still have the original rear height/load sensing valve (mounted to the rear frame, above the rear axle on the left side)? If so, is it functional? Note -- if it's there and working, there would be TWO brake lines from the front to the rear of the truck.

 

 

[schardein]

30 minutes ago, Eagle said:

 

Thanks Eagle, i always forget about the rear prop valve.

[drcomanche]

45 minutes ago, Eagle said:

 

In the MJ, that block up front (which it would appear drcomanche doesn't have) is NOT a proportioning valve. It offers NO proportioning function whatsoever -- it's a distribution block, plus the device that triggers the brake failure warning light on the dashboard.

 

Pickups especially need some sort of proportioning valve because when the bed is empty it's VERY easy for the rear brakes to lock up before the fronts, causing a spinout. But -- when the bed is fully loaded, you need all the braking you can get on the rear. This is why the XJ has the proportioning function built into the front distribution block, but the MJ uses a rear proportioning valve that responds to how heavily the bed is loaded by increasing the braking to the rear wheels under load.

 

The first question is: does the truck still have the original rear height/load sensing valve (mounted to the rear frame, above the rear axle on the left side)? If so, is it functional? Note -- if it's there and working, there would be TWO brake lines from the front to the rear of the truck.

 

 

I forgot that the mj had a load prop valve. In my case, there is no valve. All I had was an adjustable valve in the engine bay with a single line to a t fitting on the rear. It makes sense for me to have it, since mine doesn't haul anything but a spare and a fuel cell. It's strictly for fun, so it won't be filled with anything more than that.

[Eagle]

21 minutes ago, drcomanche said:

I forgot that the mj had a load prop valve. In my case, there is no valve. All I had was an adjustable valve in the engine bay with a single line to a t fitting on the rear. It makes sense for me to have it, since mine doesn't haul anything but a spare and a fuel cell. It's strictly for fun, so it won't be filled with anything more than that.

 

Put an adjustable proportioning valve up front, near the master cylinder and the distribution block, and then at least you can manually dial in more rear brakes if you should ever find yourself hauling some load.

 

Question: Do you have the original distribution block/warning light sensor, as shown in my photo above? If not, I would strongly advise you to get one and install it while you are re-doing the brake system. If you can't find one for an MJ, get one from a junkyard XJ. Remove that plug from the nose, throw away the spring and the O-ring from the plunger, push the plunger all the way into the "nose nut" (for want of a better term), and you'll have a brake warning light with no proportioning function.

[drcomanche]

38 minutes ago, Eagle said:

 

Put an adjustable proportioning valve up front, near the master cylinder and the distribution block, and then at least you can manually dial in more rear brakes if you should ever find yourself hauling some load.

 

Question: Do you have the original distribution block/warning light sensor, as shown in my photo above? If not, I would strongly advise you to get one and install it while you are re-doing the brake system. If you can't find one for an MJ, get one from a junkyard XJ. Remove that plug from the nose, throw away the spring and the O-ring from the plunger, push the plunger all the way into the "nose nut" (for want of a better term), and you'll have a brake warning light with no proportioning function.

There was no other brake components on it since I've had it, so no distribution block or anything. I figure I'll grab the xj model which does rear brake distribution at the block vs the mj which does it at the load prop valve and then also have a dialed valve for the rear. I'll try to get one with a sensor, as having a warning light would be useful.

[Ωhm]

9 minutes ago, drcomanche said:

I'll try to get one with a sensor, as having a warning light would be useful.

Plus you'll get Brake Warning Light Bulb Check during CRANK and Brake Warning Light if Parking Brakes are ever applied.

[Eagle]

2 hours ago, drcomanche said:

I figure I'll grab the xj model which does rear brake distribution at the block vs the mj which does it at the load prop valve and then also have a dialed valve for the rear.

 

 

Do NOT run two proportioning valves. VERY bad idea, because you will have no way of controlling what the combination is doing. If you use a combo valve from an XJ (or a ZJ Grand Cherokee), open it up and disable the proportioning the way I described. Either that, or don't use an adjustable proportioning valve in series with the XJ proportioning valve.

 

Quote

I'll try to get one with a sensor, as having a warning light would be useful.

 

All XJ combo valves have the brake failure warning system built in. Look at the photo I posted above. That horizontal thing running across the top of the device is a shuttle valve.The plastic thing sticking out of the top is the switch. When the plunger is down (as in the photos), it rests in that narrow section of the shuttle plunger. If the plunger moves either direction (depending on whether the front or the rear brakes fail), the plunger is pushed up to close the circuit and activate the light on your dashboard.

 

[Jeep Driver]

I have rear discs, no prop valve, brakes are perfect. 

 

 

[schardein]

11 hours ago, Eagle said:

 

Do NOT run two proportioning valves. VERY bad idea, because you will have no way of controlling what the combination is doing.

I disagree here. In my first post I describe doing exactly this. The adjustable valve gave me the ability to further reduce what the stock valve was already doing.
 

But since the OP is starting from scratch, this might be the best solution:

 

https://www.wilwood.com/MasterCylinders/MasterCylinderProd?itemno=260-11179

 

[Eagle]

1 hour ago, schardein said:

I disagree here. In my first post I describe doing exactly this. The adjustable valve gave me the ability to further reduce what the stock valve was already doing.
 

But since the OP is starting from scratch, this might be the best solution:

 

https://www.wilwood.com/MasterCylinders/MasterCylinderProd?itemno=260-11179

 

 

I'm familiar with that unit, but it doesn't provide any way to operate the brake system warning light.

[schardein]

3 minutes ago, Eagle said:

 

I'm familiar with that unit, but it doesn't provide any way to operate the brake system warning light.

It's funny, I was trying to remember why I decided against putting one of these in my CJ.  Exactly this reason.  Every time I look at the picture, I think that brake switch is a warning light circuit.

 

Also, isn't it true that even without the brake failure warning light switch present or plugged in, the brake warning light on the dash still functions to indicate the parking brake is on?  I know that's the case in my CJ.  Haven't tested it on my XJ or MJ.

[Ωhm]

14 minutes ago, schardein said:

Also, isn't it true that even without the brake failure warning light switch present or plugged in, the brake warning light on the dash still functions to indicate the parking brake is on?  I know that's the case in my CJ.  Haven't tested it on my XJ or MJ.

 

MJ uses a two (2) pin connector. Note jumper between terminals on the switch. This jumper continues the grounding leg for the Brake Warning Light to the IGN SW (bulb check) and the Parking Brake SW.

 

 

[Eagle]

2 hours ago, schardein said:

 

Also, isn't it true that even without the brake failure warning light switch present or plugged in, the brake warning light on the dash still functions to indicate the parking brake is on?  I know that's the case in my CJ.  Haven't tested it on my XJ or MJ.

 

I believe that should be the case, but I would still want the brake system failure light to be operational. Although I would hope that anyone who is a member of this forum knows enough to be able to tell when their brakes have filed without a light to confirm it.

[schardein]

9 hours ago, Ωhm said:

 

MJ uses a two (2) pin connector. Note jumper between terminals on the switch. This jumper continues the grounding leg for the Brake Warning Light to the IGN SW (bulb check) and the Parking Brake SW.

[AZJeff]

22 hours ago, Jeep Driver said:

I have rear discs, no prop valve, brakes are perfect. 

 

 

Same here.  I was uncertain how the balance would be since I already had no original MJ load sense valve.  I found that swapping to discs resulted in surprising good balance.

[Minuit]

For what it's worth, I also have rear discs but with the prop valve still in place, and still using the MJ distribution block and single diaphragm booster... you know, what everyone says not to do when you convert to rear discs. The world didn't catch on fire, and my truck stops just fine.

 

The '91 and '92 models have an adjustable prop valve, so I could adjust for proper brake bias after the conversion. When I still had rear drums, I used that to turn up the rear brake pressure a little, which made things better, but my drums weren't working at 100% (are they ever?) before I dumped them for discs. After the disc conversion, I immediately had issues with premature rear lockup. After readjusting the LSPV for correct brake bias, I ended up very close to the original position.

[Eagle]

Considering the rear disc brakes, the ZJ factory combination valve would probably be a better choice than an XJ combination valve. The ZJ unit is calibrated for disk brakes.

 

[1989 comanche mj]

I have rear discs, the zj prop valve with a single line back, stock mc and booster, got rid of the height valve and it works well with big tires.