I have a 1979 F150 2WD Automatic no CC no AC.

There is an intermittent brake problem as follows.....

Go out some days and brakes act completely as normal. Good load balance between front and rear. Normal pedal travel without pulsing. No sponginess. Stopping distances totally normal.

Go out another day and perhaps for first part of trip rear wheel lockup occurs and pedal is really hard as if no power boost. While on same trip problem may self-correct and return to normal mode as described.

Having read through this the greatest site in the world I have leaned towards the problem being caused by some blockage or malfunction within the Proportioning Valve. I see it as letting pressure through to the rear but near complete blockage to the front during malfunction. This is perhaps being caused by the piston or slug being snagged on corrosion or debris in my theory.

Manifold Vacuum is totally normal (I have vacuum gauge in cab) and check valve at booster is functional and very good vacuum is being drawn. No hiss hiss sound from the booster ever. No leakage bypass pedal sinking problem being manifested by the master cylinder. No loss of fluid or leaks.

My questions are as follows:

1. Do you agree with my diagnosis that problem is being caused by the PV??

2. If you do not agree with my diagnosis what do you suggest is the issue??

3. If it is the PV what can I do?? Is there a rebuild kit available?? Is it restorable?? I see a new one is not easy to obtain and is expensive.

4. I have a PV on another truck -a 1979 F350 which I am not using. Would that PV be interchangeable to the F150 for a test and/or permanent??

 

Chances are it is malfunctioning after years of use and possibly rusting inside from sitting for an extended time (if it has)

 

1975/79 F100/150's & F250's w/6200 GVWR (both with/without 4WD) AND 1976/79 F250's w/4WD & 1979 F350's w/4WD: D5TZ-2B257-B.

1974/79 F250's w/o 4WD w/6900/7500/8100 GVWR & 1974/79 F350's (exc 1979 F350's w/4WD): D4TZ-2B257-A.

D5TZ2B257B .. Proportioning Valve is still available from Ford.

Ford suggested retail price: $208.98 / ftepartsguy.com price: $150.47.

 

Those units are not user-serviceable except for the brake light switch.

It's possible the PV valve is going south, but I'd suggest checking the rear brakes for any weak hardware (return springs and such) and it wouldn't hurt to flush out the brake fluid and renew it. Look at it this way--if you replace the valve you're going to have to flush/bleed it out anyway. A good flushing out might just flush out any bits of crud that could be bunging up the valve. (words of wisdom: Soak the heck out of the bleeders with PB blaster. Not so much that it gets on the pads and shoes---but enough to make sure the bleeders do not snap off when you do this)

S-

 

Thank you all so much for the help. I went and examined the PV on the F350 and F150. The main difference I see is that the one on the F350 has a Push-In Bleeder pin whereas the PV on the F150 has a Pull-Out bleeder pin.

Ignoring this difference surely these PVs are interchangeable????

 

 

I was looking at an adjustable valve from Summitracing and wondering about the comparison to a factory one. I don't want to use a used one or two that I have that have been sitting for quite some time. What do ya thinK???

 

Just for kicks and giggles, check the star wheel adjusters. I had a similar problem, except I don't remember a hard pedal. However, when the rear occasionally locked up, it was always the same wheel. The teeth were worn flat on the other side. Doesn't sound exactly the same, but better to look before spending money.

 

Clarification: This is not the brake (stop) light switch.

This is the low brake fluid warning light switch. It screws into the brass proportioning valve.

C8AZ-2B264-A ~ 1973/79 F100/350 ~ Still available from Ford.

Ford suggested retail price: $29.67 / ftepartsguy.com price: $16.02.

The switch is plastic and here's what happens to it:

Minute cracks develop around the threads, brake fluid seeps out, the low brake fluid warning light on the dash comes on...and stays on.

 

Have you had the rear brakes apart to look at them???? This sounds more like oil on the brake linings more than anything else. If the axle seal leaks even a small amount of oil and it gets on the linings, it will cause the wheel to lock up when the brakes are applied. It tends to stop acting up after the vehicle has been driving for a few miles and the brakes gets warm. The colder it is the worse it will act, I have had to repair this very problem on all makes and sizes of trucks right up to Tractor trailers and it is always the same symtom as you are having.

 

Hmm I wonder if this is my problem. My light stays on also. Glad you posted that, I will have to go check.

 

Excellent posts from everyone. Let me throw my 3 cents in on this. I own a 77 F-150, 2WD, that exhibited some very similar symptoms to what dionysius described. I live in the hot southwest and had good performance from the braking system on this truck with the appropriate care and maintenance over the years I've had her.

The problem didn't show up until I replaced the original power booster with a rebuilt unit from Auto-Zone about 4 years or so ago. When the weather turned cool, I noticed that I had basically lost the power brake function for the first few minutes or so after firing it off and driving away. I got used to the new personality and figured I had a bad check valve or wrong pushrod adjustment on the booster, etc.

Fast-forward to today. I just recently completed a re-build/re-furbish of the entire braking system. Starting with the power booster and going all the way to the rear brakes. New hoses, pads, shoes, master cylinder, etc. I rebuilt the calipers and wheel cylinders, flushed all the lines. Replaced the power booster-got a rebuilt replacement free from AZ-love that lifetime warranty.

I became intrigued with the Proportioning valve and thought I'd see what I could do with it. After carefully removing it and cleaning it off, it really wasn't that hard to disassemble. The trick was being able to remove the sliding piston/valve assembly from the main body - not to hard if you're VERY careful. Long story short - these things are grunge collectors and it's easy to see why they can cause so many problems.

The pushrod of the brake switch was corroded and stuck in place in the valve, so that has to be replaced (Thanks for the part # NumberDummy). The bore and piston/valve assembly was sludged up from years of replacement rebuilt master cylinders that didn't seal well. Lesson-learned.

Bottom line - just got it all back together and bled. The system seems to work quite well, but I need to give everything time to seat and wear in before the final verdict.

I regret not taking any pictures of the valve, but the o-rings around the piston/valve assembly were in surprinsgly good shape-probably because the valve had frozen in place from the junk and the o-rings were well preserved in the fluid around them. Just took lots of brake cleaner and fluid flushing to clean all the crap out of there. I inspected the rings and could not see any scratches, tears - they were in great shape. It took about 2 hours to thoroughly scrub and clean the inside of the valve.

Caveat-I DO NOT recommend anyone to try and to clean the valve as I did. Do so at your own risk. It is a job that requires a lot of patience, time, care and brake cleaner. I figure I just got lucky (so far). Ford says they are a non-serviceable item for a reason. I will spend a lot of time away from traffic for the time being while I continue my evaluation of the rebuilt brake system.

Again - great posts. That's what makes this such a great site. Thanks

Jim