proportioning valve

After replacing EVERYTHING but the proportioning valve, my rear shoes on my 87 XJ still wont lock up (rear axle on stands and tranny in gear). I know that the proportioning valve reduces pressure to the rear brakes, but if it were bad could it reduce the pressure to much and keep my rears from applying at all.

BTW everything is bleed and adjusted correctly.

Thanks

maybe you just need to ajust the brake shoes outward so when you step on the pedal, the shoes go further outward and actually stop the vehicle.
i'd either say its a bad shoe ajustment or a bad proportioning valve.

Proportioning valves only come into play during panic stops basically.

Whats the scoop on using/not using adjustable proportioning valves on Jeeps?

Ken (dummy whos just wondering)

The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes. The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located. If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. "If" the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.

Right! So, wouldnt using an adjustable proportioning valve on a Jeep be beneficial for say going from smaller to bigger tires? Or for major off roading and reset for street? I know many rodders that use them and was just wondering if their would be any benefits on a Jeep? Lets say for example a stock Jeeps prop. valve is set at 60/40 (60 front) If you add an 8 3/4 rear end that has 11 1/2" drum brakes wouldnt you need to decrease the proportion to the rear a little so they wouldnt lock up and wouldnt an adjustable valve be the way to go? I dont know the answer and was just wondering out loud.

Thanks,
Ken

I was planning on an adjustable vavle in my MJ, but that's more for when I carry a load in the back, and since the bed height propo vavle has been disabled then I need something back there.

I just put new rear brakes on my '87 today. Even with a D44 the rear brakes are tiny - 10" x 1 3/4, so I don't think you should expect much from them.

Mine had been working poorly for some time, grabbing and squealing but not doing much stopping. I was surprised to open it up and find at least half the lining remaining, drums looking just fine, and wheel cyls just beginning to leak but perhaps siezed on the trailing side.

I put on some semi mets and new slave cylinders and hardware and just bead blasted the glaze off the drum surfaces. I was a loner at the shop today so I had to gravity bleed and I'm surprised at how well that went. So now with all the nice new stuff my heep's brakes are stopping it about as well as any other (which is nothing to write home about). I tested off the ground and with some effort they would kill the idling engine in 1st gear, and now the parking brake handle only pushes the cupholder up about 3/8".

As for locking them up, I doubt it would be very easy to get them to do that on dry pavement, but I can't imagine why you would want them to. My old S-10 had the same axle but the 2-1/2" brakes and lockup was a problem with it. The Jeep has more weight on the rear and would be just fine with the larger brakes but that isn't what they put in them. Still, if you start messing with proportioning valves you will likely find that point where they lock up and that will probably come before appreciably improved stopping power. Best bet if you want really improved brakes is to go rear disc. I heard somewhere if you have a D44 you can get the parts off a Crown Vic at the junkyard and they go on pretty easy.

The proportioning valve on xj's (and zj's not shure about mj's) are 50/50 split except at high pressures (hard braking). It's possable that the spring loaded valve inside stuck causing the uneven ratio split for high pressure (I don't know what it is). What I would do is just replace it, and do a full fluid change. Brake fluid is is hydroscopic, and will retain moisture causing lower boiling points, change the working pressure of the system, and corrosion in the system.

I had an issue very similar - I had very poor brake performance. Changed the master cylinder, proportioning valve, big round thing on firewall, new hoses...nothing! It turned out to be a faulty wheel bearing up front....the caliper moved the wheel and all the pressure wwent there. So check the front wheel bearings...mine's SO MUCH BETTER now.

Lonestar -

It would have helped if you mentioned the year of your XJ and how many miles, but nonetheless --

The stock proportioning valves do get gummed up and stop working. The FSM says they cannot be serviced and have to be replaced, but I know of people who have cleaned them and they worked again. I'd say the proportioning valve has about a 95% probability of being the cause of your problem. One of my friends in the NAC had the same problem. He removed the OEM proportioning valve and used a Wilwood adjustable unit. He has all the rear brakes he could ever hope for now.

The problem is that the OEM proportioning valve also activates the brake system warning light. Removing it is probably illegal. If you remove it, your warning light will not activate of you lose pressure in the front or rear circuit. That's probably not an issue for most of us, as we would notice the difference in feel instantly, but in most states you could never sell the vehicle that way if they make you sign a statement that the vehicle is legal for highway operation when sold. Plus if the buyer ever had an accident and an investigator found that the brake warning system had been disabled, it could come back on you.

IMHO the adjustable proportioning valve is a good solution for a trail rig/trailer queen, but for legal and liability reasons it's not a good solution for a street vehicle/daily driver.

BTW -- Did "everything" include the rubber hose from the rear chassis to the rear axle? If not, the hose may be deteriorated internally and blocking flow.

Eagle said:

It would have helped if you mentioned the year of your XJ and how many miles, but nonetheless

Click to expand...

Its an 87 with 200,000 on it.

Eagle said:

BTW -- Did "everything" include the rubber hose from the rear chassis to the rear axle? If not, the hose may be deteriorated internally and blocking flow.

Click to expand...

I replaced it with a s.s. line about 3-4 years ago. Is it possible for the nylon/teflon in the s.s. line to deteriorate. Also, how do you go about cleaning the proportioning valve if they are not servicable.

Bryan

I didn't say it isn't serviceable. I said the factory service manual says it cannot be serviced. That means they don't want the techs to waste time cleaning it when they can replace it in less time and charge you an exhorbitant price for a new part. There are a lot of parts on modern vehicles that the factory manuals say are not serviced in the field, yet you can buy parts to service them from the auto parts stores, and the aftermarket manuals like Haynes and Chiltons tell you how to service them. The manufacturers would rather have you spend your money on parts than labor -- DaimlerChrysler doesn't make anything if a tech at the dealer spends an hour cleaning your old valve.

There's a plug in the "nose" of the valve body, behind which is a plunger with an o-ring under spring pressure. You might be able to clean it by removing the plug in place (carefully -- that spring is rather strong) and squirting brake cleaner in there, and using something like cotton swabs to try to get at any gunk. The better way, of course, would be to remove it from the vehicle and take it apart on the bench, so you can be sure you have all passages cleaned out. Poking a thin wire into each outlet will help ensure that all passageways are clean.

Either way, you'll have to bleed the brakes after removing the plug because you're going to lose the fluid that's in the valve body.

Thanks for the advise Eagel. I'll give it a try, since the dealer wants $91 for a new one.

Bryan

I just finished reading the thread that was linked. Thanks! I still have questions though. I will ask here since I am askin about altering the valve or removing it. What does removing the o-ring do. Will it help any to remove it? I am going to pull the valve and clean it and I will remove the o-ring at that time if it might help.

BTW, i have 36" tires with probably 350 lbs of weight in the rear including bumper, cage, spare tire and parts, etc. I know that this is probably a factor.

I do see discs in the future for the rear but for now I want to try tweaking what I have and see the results.

Eagle said:

The problem is that the OEM proportioning valve also activates the brake system warning light. Removing it is probably illegal. If you remove it, your warning light will not activate of you lose pressure in the front or rear circuit. That's probably not an issue for most of us, as we would notice the difference in feel instantly, but in most states you could never sell the vehicle that way if they make you sign a statement that the vehicle is legal for highway operation when sold. Plus if the buyer ever had an accident and an investigator found that the brake warning system had been disabled, it could come back on you.

Click to expand...

The PO of my XJ said the brake light had been on for the last 50k miles, its probably a short. I asked if he noticed the brake pedal level was almost to the floor, he hadn't, and he was surprised when we opened the brake reservour and it was dry. The rear brakes were leaking and that stage of the brake system was dry and completely unused. Geeez, your right Eagle, there are guys out there that need a brake light, and its sad that even an idiot light still doesn't clue people in that their brakes are unsafe.

I'm still surprised no one makes an aftermarket sensor for use with the adjustable proportioning valve. Its a real simple device.

Is it possible to gut the proportioning valve portion of the combination valve and thus still use its warning light function, but no pressure drop, which you use the adjustable proportional valve after it to do that function?

Dave in Eugine - "Even with a D44 the rear brakes are tiny - 10" x 1 3/4" ...

I believe the XJ D44 uses 10" x 2 1/2" shoes, no?

GM

OK, a diagram of a Combination valve, granted its an oldsmobile:

and a pic of another disassembled, not sure if its a oldsmobile, but it looks like the diagram:

Look like, to me, you can modify a combination valve so that you'd defeat the proportioning valve in the combination valve, then just splice in your adjustable proportioning valve. Hopefully the pressure differntial piston would still work properly.

Even if the Jeep/Chrsyler combination valve is unable to be dissassembled, since you dumping the proportioning valve anyway, you could just get an oldsmobile proportioning valve and modify it. Doesn't matter if its an oldsmobile, the pressure differential piston, is suppossed to stay put unless one stage fails and there is difference in pressure between the two stages. Both stages are working properly, pressure between them will be even, and the piston stays put. One stage fails, the pressure between them will be different and the piston is forced one direction or the other and that will throw the switch to light the light.

Simple device should work the same on all cars, as long as the proportioning valve is disabled.

Rick, that Oldsmobubble diagram isn't even close to what an XJ proportioning valve looks like inside. I would suggest ignoring it, unless you're working on an Oldsmobubble.

Yes, it is possible to gut the OEM valve and retain the warning function. Remove the capped plug from the "nose" of the valve body. Take out the coil spring, remove the rubber seal (most folks call it an o-ring, but it's really shaped more like a cup) from the aft end of the plunger, push the plunger as far forward into the capped plug as it will go, and reassemble.

You will now have NO proportioning, which means full braking to the rear wheels at all times. For people who know how to drive (and for olde phartes like me, who grew up before proportioning valves were invented) this should not be a problem, but under certain conditions it may result in premature lockup of the rear wheels, which means "spin-out." I can deal with that, but I wouldn't want it in any vehicle my wife might ever drive.

Adding a manual, adjustable proportioning valve downstream should address that issue -- as long as it is correctly adjusted. Another work-around is to use a smaller diameter than stock rear wheel cylinder to reduce overall braking at the rear wheels.