Brake distribution (proportioning) valve rebuild?

[Eagle_SX4]

To start this off a few months ago when I changed all the rotors/pads and drums/shoes/cylinders. I ran into an issue bleeding the brakes. Following the procedure from the manual of bleeding the rear brakes then the fronts then opening the fronts to force the shuttle valve to move to bleed the emergency bypass line. I could never get the shuttle valve to move. I did remove the switch from the valve and tested its functionality and with it removed I could turn on and off the brake light on the dash. Proving that the shuttle was not moving even with both front caliper bleeders opened.

 

I do know that my height valve functions as intended with the wheels off the ground I moved the lever to different positions simulating a full bed and and empty bed. The required pressure on the brake pedal to lock up the wheels did change.

 

Another strange thing was happening. This may or may not be related to the distribution valve. I had rear brake pressure when the fronts were hooked up but not when the front bleeders were open. Meaning that I would have no brakes if something were to happen the front brakes. This is the main reason I am looking in to rebuilding/cleaning the distribution block.

 

I am thinking about rebuilding it. Has anyone replaced the o-rings inside of the valve? I know the service manual says to not attempt a rebuild. They have not been available for a long time and everyone seems to have a different opinion on replacement/deletion of the height valve and distrubution valve. 

 

From the pictures shared by @Eagle it looks like it would be easy.

You would just need to find the correct size and type of o-rings. From my limited research it looks like EPDM would be the correct material.

 

I am just trying to figure out the mystery surrounding the proportioning valve.

[fiatslug87]

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Sorry, I’m no help but very interested in what you come up with.

[gogmorgo]

I'm also curious about a rebuild. There's really no reason you shouldn't be able to do it. The things to watch out for would be getting the correct material o-rings correctly sized, and correctly seated. You also don't want any debris getting in there. You'd want to watch for any erosion on machined seal surfaces as well. I strongly suspect the main reason the service manual says not to is because the cost of a new one is small compared to the risk of something going wrong during a rebuild. I'd recommend lubing the o-rings with brake fluid before putting them in.

 

I recently had my valve shuttle itself during a panic stop. I blew a rear line. The light came on, the pedal got soft, but with the valve fully shuttled the rear lines are fully blocked and no longer leaking. Looking at the cutaway, I think that's supposed to happen. I haven't actually addressed the issue yet, so no real experienced advice on what to do.

But I will point out that every procedure I've seen until a very recent post here said to open the front bleeder to simulate the front failure first, bleed the rears, close the front bleeder and bleed the rears again, then the fronts. I wonder if having the front bleeder open initially while bleeding the rear is critical to pushing the valve in the correct direction so it doesn't shuttle the valve the wrong direction at the wrong time and block brake flow in a way that's difficult to correct? You'd think pushing the pedal with both front bleeders open would be enough to overcome that, but it's possible it's not. I wonder if cracking the brake line on the master cylinder and removing the pressure from the functioning circuit would be enough to shuttle the valve back the other direction and unblock the line?

I've never actually paid attention while bleeding to see if the light comes on indicating the valve shuttles. As in I've never done it with the key on. I've also never done a proper bleed on this particular truck that popped a rear line, I don't think the rear brakes have ever worked correctly for it and I've never made much effort to make them do so. But I guess I have to now.

[GonzoTheGreat]

I will admit that my understanding of this valve is a little hazy, but from the pictures I see no reason why it wouldn’t be rebuildable. 
 

12 hours ago, gogmorgo said:

 

I recently had my valve shuttle itself during a panic stop. I blew a rear line. The light came on, the pedal got soft, but with the valve fully shuttled the rear lines are fully blocked and no longer leaking. Looking at the cutaway, I think that's supposed to happen.

 

I will also share that I’ve blown a rear line and had the shuttle block off the rear. I didn’t lose all my fluid, and although the pedal was soft, I still had front brakes. I fixed the line and bled the rear, I gave the pedal a final stomp just to feel if there’s any air in the lines, and when I turned the truck back on, the shuttle had returned to center and I didn’t have the light on on the dash. I never touched the front brakes during this process. 

 

16 hours ago, Eagle_SX4 said:

Another strange thing was happening. This may or may not be related to the distribution valve. I had rear brake pressure when the fronts were hooked up but not when the front bleeders were open. Meaning that I would have no brakes if something were to happen the front brakes.

Is it possible you didn’t slam the pedal hard enough to move the shuttle? I have never bled according to the book, so maybe someone else can speak for how much pressure it takes to move the shuttle while bleeding, but for me I was basically standing on the pedal after my line blew. 

[Eagle]

On 9/17/2024 at 7:11 PM, gogmorgo said:

I recently had my valve shuttle itself during a panic stop. I blew a rear line. The light came on, the pedal got soft, but with the valve fully shuttled the rear lines are fully blocked and no longer leaking. Looking at the cutaway, I think that's supposed to happen. I haven't actually addressed the issue yet, so no real experienced advice on what to do.

 

 

That's the way the system is intended to work. If the front brakes fail, the shuttle moves to allow the system to bypass the rear proportioning valve. If the rear brakes fail, all the shuttle does is turn on the brake system warning light.

 

Quote

But I will point out that every procedure I've seen until a very recent post here said to open the front bleeder to simulate the front failure first, bleed the rears, close the front bleeder and bleed the rears again, then the fronts. I wonder if having the front bleeder open initially while bleeding the rear is critical to pushing the valve in the correct direction so it doesn't shuttle the valve the wrong direction at the wrong time and block brake flow in a way that's difficult to correct? You'd think pushing the pedal with both front bleeders open would be enough to overcome that, but it's possible it's not. I wonder if cracking the brake line on the master cylinder and removing the pressure from the functioning circuit would be enough to shuttle the valve back the other direction and unblock the line?

I've never actually paid attention while bleeding to see if the light comes on indicating the valve shuttles. As in I've never done it with the key on. I've also never done a proper bleed on this particular truck that popped a rear line, I don't think the rear brakes have ever worked correctly for it and I've never made much effort to make them do so. But I guess I have to now.

 

Tripping the shuttle is necessary in order to bleed the bypass circuit in the rear brakes. If you don't trip the shuttle, bleeding only bleeds the normal (proportioned) part of the rear brake system, which may leave air in the bypass circuit. That would not be a good thing.

[gogmorgo]

On 9/21/2024 at 8:42 PM, Eagle said:

 

Tripping the shuttle is necessary in order to bleed the bypass circuit in the rear brakes. If you don't trip the shuttle, bleeding only bleeds the normal (proportioned) part of the rear brake system, which may leave air in the bypass circuit. That would not be a good thing.

Well OP did apparently bleed both. But the suggested order is to bleed the bypass first and then bleed everything else. The OP suggests they bled everything first, then opened the front bleeder to bleed the rears, and is now having problems getting fluid out of the bleeders. I was postulating that having done the bleeding out of order may have shuttled the valve the "wrong" direction for bleeding and now getting it to go back the other way is causing problems.
Of course it's also entirely possible that the master cylinder is shot and not pumping fluid as it should, or some debris got pulled in from the reservoir and blocked something. Or the rubber hoses are disintegrating. All we really know is that the brake fluid is not doing what it's "supposed" to be doing. We may be entirely overthinking the problem with the distribution block, and the real solution is just to start at the top and figure out where we have the correct pressure/flow and where it stops being correct.