Brake Bleeding procedure with Rear Height Sensoring Valve

[05mxdiesel]

I wish I would of saw this a few days ago after replacing calipers, pads, rotors, and all the rear lines. If you do any kind of brake work in the rear that involves opening up the system, please read this. I spent about two hours trying to bleed lines, while getting screamed at by my wife who was working the pedal. Everytime I went back to the rear I was getting air out of the lines. Thanks to who posted this before.

 

 

Yeah, the rear proportioning valve DOES require a special procedure. First, bleed all four wheels normally. After that, the FSM says to do the following:

 

Quote:

After bleeding the brake calipers and cylinders the rear brake by-pass line must also be bled.

 

Open a front caliper bleed fitting and depress the brake pedal to the floor. This will shuttle the by-pass differential valve and allow fluid to flow through the by-pass line. The brake warning light on the instrument panel will illuminate when the ignition key is in the ON position. This signals the shuttling of the valve.

 

Re-Bleed the rear brake cylinders with the front caliper bleed fitting open.

 

After re-bleeding the rear brake cylinders the entire system must be bled again.

 

Bleed the brake calipers and cylinders in the following sequence:

 

1st -- Right rear

 

2nd -- Left rear

 

3rd -- Right front

 

4th -- Left front

[91coMANche]

Oh man! I just got my Dana 44 in and will be bleeding in a bit! Thanks for the heads up!

 

Can you clarify how to rebleed the rear with the front bleeder open? If I understand correctly I bleed the entire truck. Then open a front bleeder and push the pedal to the floor...do I think close the bleeder before releasing the pedal? Now you say to bleed the rear with the front open...wouldn't that allow air into the front? Once I bleed with the front open, do I close the front and rebleed the rear?

[big66440]

What if you're using a pressure bleeder?

[Eagle]

What if you're using a pressure bleeder?

 

You still have to open a front caliper and blow the pressure off the front circuit so the shuttle valve will open the by-pass circuit to the rear. Otherwise, the pressure bleeder can't push fluid through the second line.

[big66440]

do all comanches have this? I'm planning on doing a huge overhaul on mine in two weeks?

[Eagle]

do all comanches have this? I'm planning on doing a huge overhaul on mine in two weeks?

They all did when they left the factory. I know of one 1988 red Chief that doesn't have one now ... and I know of two others that won't have it when they get running and registered.

[big66440]

so what exactly does this valve do? keep the back from locking up? give pressure to the drums before the front discs?

[flint54]

The height sensing valve meters fluid to the rear brakes. As the distance between bed and axle

decreases (due to added load in bed or on hitch), the valve permits increased fluid flow to the

rear brakes. Some folks believe this valve is; A) not necessary, and/or B) an added risk.

For point A, folks contend that good drivers can manage their braking just fine when carrying

a load, and point to the fact that most other trucks have no such valve. For point B, folks

contend that the valve is most likely quite old and subject to sudden failure under emergency

braking pressure. I can tell you that the valve is not serviceable, and new replacements are

not available. Mine remains installed because it works fine and it preserves originality (a fact

that is becoming less important to me as I continue to upgrade with components from other

Jeep models). My opinion is that the failure risk is real, but low probability. I believe the most

probable failure mode would be a bypass flow due to wear of the internal seals, and the result

would be an increase to the metered flow rather than a total and sudden loss of system pressure.

But, until a failure occurs, nobody knows. If you're concerned about any of this, the valve

can be very easily removed. Search the archives, much has been written about this.

[Eagle]

so what exactly does this valve do? keep the back from locking up? give pressure to the drums before the front discs?

Flint54's explanation of how it works is pretty good. The reason for it is that a pickup without a load has very little weight over the rear axle, so it's easy to have the rear brakes lock up before the fronts. The height-sensing valve is intended to alleviate this, while adjusting the rear brakes to provide more braking when the bed is loaded. And, if properly adjusted, it does that very well.

 

I am only partially in flint's group A. I don't regard the height-sensing valve as absolutely necessary, but I do regard it as a good thing. I am, however, in flint's group B. I have personally experienced a potentially catastrophic failure of a rear height-sensing valve. I had to make a panic stop, and the valve simply exploded. The result, of course, was NO rear brakes. So I don't trust them.

 

If they were available (and not overly expensive) I would replace it. But they have not been available for many years. Personally, I am comfortable driving without one. It must be recognized that, under some conditions, the rear wheels WILL lock up prematurely with no proportioning to the rear wheels. I grew up long before such devices were offered, so this doesn't bother me. For those who aren't accustomed to dealing with rear brakes that lock up before the fronts (which can cause oversteer and spin-outs), I recommend removing the height sensing valve and installing a Wilwood (or Mopar, which is probably the same thing) adjustable proportioning valve in the rear brake circuit. It can be set to properly limit the rear brake force when empty, and then allow more rear brake force when loaded. The downside is that it's not automatic -- you have to experiment, find the right settings for your truck, and remember to turn the knob when you load or unload your truck.

[big66440]

Ok I replaced the front right brake hose and both wheel cylinders,I'm using my pressure bleeder and my strategy was this 1,I left the truck at curb height (didnt jack it up or put it on stands) then bled the entire system RR,LR,RF,LF. 2 I turned the switch to the "on" position,opened up the RF bleeder and bled the two rear wheel's RR then LR. I fired up the truck and there's very little pressure and the pedal goes to the floor, I noticed I forgot to rebleed the entire system after step 2,is that really necessary?

[Eagle]

After you bled and then opened the front bleeder, did you step on the brake peddle? The purpose of opening the front bleeder is to simulate a front brake failure, causing the shuttle valve in the front metering block to move and open the passage to the bypass circuit to the rear. If you're using a pressure bleeder, I don't think just opening the front bleeder would activate the bypass circuit.

[big66440]

Ok I turned the ignition switch to the on position and depressed the pedal and held it there with a jack handle (I had no assistant) did the procedure and it's still somewhat spongy,it holds enough pressure to stop the truck at slow speeds and no air comes out,any ideas?