Getting closer to having my 67 f100 running and driving. When I swapped to power disc brakes I also grabbed the pressure differential valve from the donor truck. Now that I'm bleeding the brakes, I find that I have no fluid going to the rear brakes. I've spent the past couple of hours searching on here and decided to just ask for help. I'm assuming the valve is stuck. Is it possible to reset it or am I just going to have to replace it? From my reading on here, the factory valves are no longer available. So that leaves me with aftermarket. If I have to replace it, what do you recommend replacing it with?

Thank you as always for the help.

 

What year and series was the donor truck?

 

Sorry, that would have been helpful. Was a 77 2wd F150.

 

Try to take it apart and clean out the old brake fluid and residue. The repro’s have a high tendency to leak.

 

D5TZ-2B257-B .. Brake Pressure Differential Valve / Marked: D5TA-2B524-FA or FB / Obsolete

1975/79 F100/150 2WD/4WD // 1975/79 F250 2WD with 6200 lbs. GVWR with single piston caliper disc brakes // 1975/79 E100/150 // 1976/79 Bronco // 1976/79 F250 4WD // 1979 F350 4WD

 

The shuttle **** does not shut off fluid flow though, so even if it was tripped off-center you would still have pressure to the rear brakes. So the blockage is either gunk related as suggested, or somewhere else.
Makes sense that it's the valve of course, since you did not have this problem (presumably) before. Unless you did and didn't notice it until now? Happens sometimes...

So being a '77 then this is a "combination valve", correct? Isn't the "pressure differential valve" just the old "H-block" style with the switch for the dash light only?
Would not a '77 have had the full complement of functions, including the delay valve (hold-off valve) and proportioning valve for the rear?

I don't think there is a way to plumb it backwards, but if there is could the hold-off valve be in the rear brake circuit and not opening up?
Actually, when they work and are the type used on lighter duty trucks (light 250's and supposedly all 150's) you can actually see the little nipple thingy move out as you push on the brakes. It's the bit you're supposed to pull out manually as you're bleeding the front and normally has zero effect on the rear brakes.
Which is why I was wondering if it is possible to plumb it backwards. The hold-off valve is known for sticking closed and blocking fluid to the front.

What else was changed? And what else was not changed?
Is the master cylinder new, or from the donor? Are the front brakes plumbed to the rear port and the rear brakes to the front port?

Good luck. Very curious what you find.

Paul

 

Thank you for the replies. Brake lines and master cylinder are new. The lines are plumbed correctly as the larger rear pot of the master cylinder is going to the front calipers and I am getting fluid there. When pulling the line from the pressure differential or combo valve to the rear brakes, no fluid comes out of the valve. The only part not new is this valve, which was sitting exposed, so I'm positive it's either just clogged with junk or is just ruined. Plan is to pull it apart and see if it's able to be cleaned or repaired. If not, then try to find another used one. Thanks numberdummy for the vehicles to look out for.

 

Let us know what you find.
And I think it was someone here recently that had a bad "new" master cylinder that only pumped out of one port. Might have been over on classicbroncos, but not sure.

Seems like we run into problems with new parts all over the place these days. Does not have to be electrical anymore either. I even had a customer that after all the tests found that something was clogging his brand new combination valve I'd sold him!
Up to that point the only problem had been leaking hold-off valves. Which to me was enough of a problem to stop selling them. But we'd kept up because not everyone wanted the alternative that I personally think is the way to go, and that's the manually adjustable rear proportioning valve.
After his experience though, I at least know to suggest it if someone has an odd issue that defies explanation.

Good luck.

Paul

 

Was a bad valve. Replaced it with Speedway part number 910-31364. Bolted right in to factory spot on the engine cross member. Only one port was the wrong thread size, but an adaptor from O'Reilly fixed me right up.

 

Nice! Glad you got it working right again. And good to know the part number and how it fits. Thanks for keeping us updated.

Any idea what was wrong with the old one? Gunked up I'm guessing, or something else going on?

Paul

 

I honestly didn't take the old one all the way apart, but the piston/plunger was stuck in place. Ordered the one from Speedway not knowing how close it would be and was super excited to see the mounting bolts and all the ports were in the factory locations.

 

Did you get the full kit with the mounting bracket? Sounds like you didn't use it and just put it back where it was.
That's fine too (and less plumbing trouble) but if you are ever plumbing a whole new system, or if you ever get an adjustable one for whatever reason, mounting it up on the master is the way to go.

Paul

 

No, just used the factory bracket since we already had all new brake lines ran to the factory combo valve.