Pressure Differential Valve Keeps Moving!

[BigFoot]

Hi,
72 F250 2x4 drums.

For the longest time I have only had rear brakes because the differential valve was tripped. Now that its getting cold I'm thinking that I need to fix the problem.
So today I ran all new brake lines to the front. I thought it better to just replace them all. I replaced everything. Master cylinder, Wheel cylinders, springs, rubber hose, line and I took out the Pressure Differential valve and removed the pin and cleaned it. I then bled the entire system til there was no more air. I had to switch from back to front bleed a few times to get the warning light to go off (to re-set the Pressure Differential valve).
I had firm pedal. I drank a beer talked with my buddy and then before I had a chance to put the wheels back on, he jumped in the truck and told me the damn brake warning light was back on!!

So we bled again real quick and it when off. We pressed the pedal slowly while I opened the bleeder until the light when off again. We thought that fixed it.... An hour later I get in the truck to go home, I hit the brakes and BAM!! The light is back on AND now my brakes barely work at all. It seems that its still rear only but now the pedal is WAY lower than before with only rears working.

I checked everywhere for leaks and I dont see any. What am I doing wrong!! Do I need to buy this: https://www.musclecarresearch.com/brake-valve-tool to use when I bleed it? Im about to get that thing and just leave it in there!!
HELP!!!

[BigFoot]

perhaps I need to bench bleed it more. Maybe there IS more air in the line??

[jimmy828]

When i bleed brakes on all my vehicles i would start at the bleeder the furthest away from MC .Such as, back right/back left/front right then front left . If it is a new MC, it is recommended to bench bleed it before install. Hope this helps.

[4F250s]

Sounds like you have a leak somewhere. Check all the lines and fittings while you have it under pressure - with someone holding their foot on the pedal. Might just need to tighten a fitting or two on the new lines.

[BigFoot]

I bled it farthest to closest. And no leaks anywhere. I have checked all fittings and line.
The only thing I can think of is to unhook the lines from the MC and bench bleed it again.

[4F250s]

I recently replaced my brake lines and couldn't see any leaks while bleeding the system. But when we put a pressure bleeder on it, a few of the fittings dripped a bit until they were tightened another 1/4 turn or so. I'm not 100% certain, but I don't think the pressure differential valve will trip just based on air in the line. It definitely will trip when there is a leak though. All that being said, you should probably go through with another bench bleed for the MC and see what happens. Good luck and let us know how it ends up.

[BigFoot]

I recently replaced my brake lines and couldn't see any leaks while bleeding the system. But when we put a pressure bleeder on it, a few of the fittings dripped a bit until they were tightened another 1/4 turn or so. I'm not 100% certain, but I don't think the pressure differential valve will trip just based on air in the line. It definitely will trip when there is a leak though. All that being said, you should probably go through with another bench bleed for the MC and see what happens. Good luck and let us know how it ends up.

Yeah, I'm afraid to tighten everything any more. It's all super tight. I bought some fittings at HD today to make a new vacuum jar for my mityvac. I'm going to bench bleed and then vacuum bleed the system tomorrow after work. I'll post my progress.

Also, why in F did ford make it so hard to get a wrench on the front drum bleeders!! I have to unbolt the line bracket just to get to it. Arg!

[youngone]

I would think it was the master cylinder needing to be bench bleed some more. It happened to me once. Some of them need lots of pumping.

Good luck...Pat.

[BigFoot]

So, I fixed it.

I took off the master and bench bled it again. No bubbles at all. It was fine.
I bled the brakes again till the light went off (valve re-centered). I made a tool out of a 3/8ths x24 bolt and I tig welded a small pin to the end just like the one you can buy from muscle car research, removed the brake light switch from the differential valve and installed the bolt.

I then decided that I better check the rear drums for adjustment before I bleed again. That was a ball buster because the adjusters wouldnt move and so I had to take the whole damn rear end apart to get the drums off to be able to loosen the adjusters. Meanwhile I found a self adjustment cable had come off because the bonehead PO had installed them wrong. I fixed all that, adjusted them, and re-bled the system. A LOT of air came out.

I think that the KEY here is to use that tool to hold the differential valve from moving. That way you can actually BLEED the system without the valve moving and preventing you from doing so. IMO this tool for the valve is a must have for bleeding brake on these trucks.

Any way, Its fine now. I locked up all four tires on the way home. The truck kinda hopped to a skid stop. What does the hopping mean? Re-adjust all of them equally? Weird though because I get a similar "Hop" when I try to peel out.

Also, my rear shoes are almost shot. So I'll be doing that in the spring....ugh.

Thanks for the input!

[youngone]

Ahh thanks for letting us know what the problem was. Glad you got it fixed.

[sargentrs]

Congrats on getting her fixed and thanks for posting it. I'm putting front disc on mine and I'll be sure to get the bleeder tool now.

[tbone6]

The truck kinda hopped to a skid stop. What does the hopping mean? Re-adjust all of them equally? Weird though because I get a similar "Hop" when I try to peel out.

It's called wheel hop. First thing to check are your shocks/leaf springs. Lots of info if you google it.

Glad you got your brakes figured out...

[cole71]

Glad it's fixed. When you put the bolt in did you remove all your fluid or just bleed it till all the air was gone? My light is on and I think I'll be doing this. Thanks

[motzingg]

thanks for posting a solution!

i'm with ya on those stupid bleeder screws, mine are already rounded off and they're brand new. maybe it has something to do with the china crap bleeder screws i have on the new wheel cylinders vs. hard OE steel that was tougher.

[BigFoot]

UPDATE:
So, the brakes worked great for the night. the next morning on the way to work they did the same thing. (pedal lower and only rear brakes working).

After thinking about this I have figured it out. Here's the deal:
I have an air bubble in the front system because I replaced all of the line. There is air trapped in the front right hard line at the top of its travel just before the rubber hose connection. AND because of this damn differential valve I have not been able to bleed it out fully. I found that after completing bleeding it the other night with my tool that I made in place in the switch bore, once I replaced the switch...The light was on. .....Meaning that I did not get the the valve back to center before inserting my tool AND because of the that I was not able to fully bleed out a bubble in the front right circuit.

While bleeding it the other night, I was satisfied with all of the bleeds but the front right wheel. That was the only wheel that I couldnt seem to get all of the air out of. (the fluid came out well enough but it was milky with fine air bubbles). This was because the damn valve was not centered. So as a result fluid would come out when I bled it but not full pressure and therefore not enough to force out the air.

Now, the next problem is that I believe that a mity-vac would be the best option to use to suck out the air, but i need to make sure the valve is centered before I insert the valve tool because if the valve is tripped it will prevent fluid from being able to travel from the resivor, to the master and then through the valve and into the offending line with the bubble.

Yesterday I drilled two holes in the top of a mason jar lid and JB welded in two 3/16 hose barbs to make a better vacuum jar than the one that came with the pump.
This weekend I'm going to get new rear shoes and hardware kits, pull the damn rear end apart again, replace them, and then try to center the valve up front (I will remove the switch and use a mirror and a flashlight to check the position of the valve beofore inserting my tool because I found that my switch may be going bad and giving false signal to the dash light) Then vacuum bleed the front and be done with it.

Now, What is really weird is that I replaced the rear lines on my 71 bled em' with no warning lights and no problems. It all went great. I didn't even know about the valve! I thought it was just a distribution block with a pressure switch. I'm willing to bet the valve on the other truck was either stuck in the center position or because I vacuum bled them from the get go and never touched the brake pedal and therefor never put pressure on the valve with air in the system...it never moved.

And, one last note. Someone on here said that the valve would not trip because of air in the line. That IMO is false. Air compresses and fluid does not. Air in the line would cause a "differential" in pressure and throw the valve.