Hey gang...

Here is the story:

Restoring My 78 f-250 supercab 4X4, I have installed all new brake lines, new (rebuilt) booster and master. I started by trying to loosen up the rear bleeders... yup... snap. So got new wheel cylinders while I was at it. Swapped them out, and started pumping the brakes. PUMP PUMP PUMP.... no pressure coming to the back. Not even air! Feeling the lines while my helper pumped the pedal, It seemed that I wasnt getting any pressure past the proportioning valve! I then removed the rearward line from the valve to find no fluid leaking from the valve!!!!! I then removed the line coming from the master to the prop valve, Fluid comes out! What gives. The front brakes seem to be pumping up, as I can see the rubber hoses moving. (yeah... the bleeders broke on the calipers too...So I will need to buy some new dual piston calipers for the front!).

I would assume that blowing air in will not do much as brake pressure will not clear the valve. Can these be serviced. I know I can buy a new one, but I almost fell off my chair when I saw the price! I paid less for the master/booster! Hell I paid less for the complete brake line kit!

 

You have to push the pin back in the valve. Then pump slowly so as not to trip it again.

 

It sounds like the differential valve inside is blocking off the rear brakes. When the rear brakes fail, the pressure of the front brakes pushes the differential valve (which is inside the proportioning valve) toward the rear of the proportioning valve, to keep pressure to the front brakes while not losing any fluid to the rear brakes. The same is true if the front brakes fail - the valve will block off the front brakes and send fluid to the rear brakes. The differential valve has a detent in it to switch the "BRAKES" light on in the dash when the valve goes off-center, indicating a difference in front and rear brake pressure.

It's not always straightforward to re-center the valve once this has happened. This is where Mark's advice comes in - the goal is to pump very slowly in hopes that the differential valve will re-center itself. If that fails, you may be able to open the bleeders on the front brakes, so that there is no front brake pressure, and pump the brakes in an effort to move the differential valve toward center again. Then close the fronts, and then bleed all four corners like you would normally. I haven't tried that before, because I've been lucky and the valve has always re-centered on its own.

 

What a bummer of a day!

Went to the swap meet and spent $20 for the pleasure of walking around the event! In other words, two hour drive each way, $20 admission, $20 Fuel, and nothing to show for it!

Then I read the replies to my problem, and see an immediate problem. The bleeders on my front calipers are broken off! Im gonna have to buy new calipers as I seriously doupt that the calipers on the truck are still good after sitting as long as the truck has over the course of this restoration.

The good news, rebuilt calipers for auto zone are super inexpensive! Bad news is they dont stock them in local stores ! So I will have to order some unless the local part store has them reasonably priced.

In the mean time I removed the proportion valve to inspect it. I tried blowing compressed air in all the ports. The front blew free from all three ports. The rear line would not blow out either way. I then figured I might be able to push the pressure diff valve back in. Only thing I could find long enough was a coat hanger, as compressed air didnt do anything. No luck with that either.

I then pulled the valve apart, carefully! Cap, retainer, spring, rod, washer. Simple. Cleaned it out. Blew compressed air, and all seemed better. Reassembled, blew compressed air. Got Trickle. Nothing impressive.

I GIVE UP!

I will be heading to the Salvage yard this week, and getting a donor proportioning valve. Or two, or THREE!!!!

Ohhhh.... and brake fluid SUCKS!!!! My freshly painted frame now has streeks of orange rust encapsulator. Yup, the eastwood chassis paint is not brake fluid safe! GRRRRRRRRRRRRRRRRRRRRRRR

Now I also have to touch up my frame!!!!!!!!

 

Today,

I went to one of the many local pick a parts, and went on a proportioning valve removal spree!

I found two 78 f150's and one 78 f250. All three are now missing proportioning valves! I noticed that on the f150 valves that the bottom front port is blocked off. I thought it was strange on the first one, but apparently its normal (excuse my ignorance as I have yet to start my F100 short bed restoration! Im still sinking on the 78 F250 Supercab project).

Any who, I paid $20 a piece for the valves. Which I guess is fair, but I expected to pay less.

So when I get out of work tomorrow I will be cleaning the F250 valve first and checking for any blockages. KEEPING FINGERS CROSSED!

From what I have read in other posts, and the part numbers/desc that "NUMBER DUMMY" has listed, the F150 valves should work the same as the F250 valve.

 

You could always go with a universal valve too. I bought one from summit and it works pretty good and also I can adjust the amount of pressure.

 

SUCCESS!!!!!!




The F250 proportioning valve I grabbed from the salvage yard blew out clean! I put it on the truck, changed the front calipers and bled the brakes. PUMP PUMP PUMP PUFF... PUMP PUMP PUMP... FLUID!!!!


One more thing off the list!