On my 79 4WD it has disc brakes on the front and drums on the back. The sensor on the proportioning valve started leaking badly. I did not know you could get this sensor separately so I ordered an aftermarket proportioning valve from Carolina Classic Trucks. I had plans of replacing 100% of the braking system but did not have time so I just installed the new proportioning valve to get the leak stopped. After replacing it I noticed a couple times that both front disc brakes did not release completely and they got very hot. Since this was happening I got my butt in gear and have now replaced 100% of the braking system on the truck including all new stainless brake lines, master cylinder, E brake cables etc. The only thing that I did not replace is the power booster. I suspected the new proportioning valve may be defective so I ordered a second one from Bronco Graveyard and installed it along with everything else. The proportioning valve ended up being the same one that I ordered from CCT. After replacing everything it all worked great for about 3 days just driving around town and back and forth to work. Then going home last night both front brakes started dragging again and then they did the same coming to work this morning. As far as I know the only thing that can make both front brakes to drag is the proportioning valve or the master cylinder which have both been replaced. What am I missing??

 

Have you checked to make sure that the calipers move freely on the mounts? Possibly even lubed them up? It could be that the calipers are binding, or rusted and are not "floating" on the mounts like they should. Is one pad worn more than the other? That alone would be a good indicator that you've found the problem.

 

If brake fluid was leaking out from under the brake warning light switch, a new switch would not have fixed the problem. The problem is one or both of the O-ring seals (located on either side of the switch) on the pressure differential valve spool are bad.

 

ultraranger--Thank You for the heads up on the leak.

78PEB--100% of the braking system has been replaced including the calipers and the pads. The new calipers seemed to move good but will double check. Just seems odd that both of the old calipers and now both of the new calipers are doing it.

Since both front brakes occasionally drag I am assuming something is not letting the fluid release pressure off of the calipers. As far as I know that could only be the proportioning valve or the master cylinder, which have both been replaced.

 

OK.....If you said the calipers had been replaced.....I missed it.....was just throwing it out there......I agree that it would either be the master or prop valve......did it just start doing it AFTER you replaced the prop valve twice?

 

I know you said you replaced "everything" - but did mention flexible brake hoses. It is not unknown for them to fail and maintain pressure after you have let off the pedal, which would cause this.

Did you follow bleeding instructions on bleeding the proportioning valve, re the pin on it? That's the only other thing I can think of.

 

If both front brakes are locking up, the problem may be in the interface between the booster output rod and the back of the MC. The output rod may be adjusted too far out, causing the brakes to drag, get hot and seize up.

How to measure/adjust the booster output rod to the MC being installed.

https://www.ford-trucks.com/forums/1...l#post16825296

 

Flexible brake lines were also replaced.

Just noticed it doing this after I replaced the proportioning valve the first time BUT I am not for sure if it did it before because I just recently bought this truck and did not drive it a whole lot before because of the original leaking proportioning valve.

Will measure the booster output rod but am skeptical because it did it with the old master cylinder also and it does not do it all the time.

 

Is the pedal binding up????? Returning properly????? Sloppy bushings at the pedal pivot causing it to droop???? Brake light switch hanging up and not allowing the pedal to return correctly????? Lots of things once I got to thinking about it...........Doesn't necessarily need to be a hydraulic problem........Not saying it isn't but......

 

I do not think it is a pedal problem because it seems to be working properly. Plus it is only the front brakes that drag occasionally. Never the back brakes and I have the back brakes adjusted fairly snug.

 

Could it be the booster?

Great point. Common on Ford Tempos, the metal retaining brackets swell up or whatever, due to corrosion. These brackets are usually bent in a curl around the hose, and if the bracket swells and pinches the hose, it will act like a check valve, favoring the direction of flow with the most pressure - which would be the "apply" side... On Tempos, I would simply take a large flat blade screwdriver, and spread the brackets where they curl around the hose.
I suppose the old brake metering/proportioning block could've been impeding pressure to the fronts, masking the problem until you noticed it and started working on it.
For whatever it's worth, my truck always felt like the front brakes weren't much applying since I got it in October. Under braking, I was feeling 'eccentric' vibrations, characteristic of drum brakes, like it was favoring the rears. If I applied normally, then lifted and reapplied again, I could feel the fronts kick in more strongly.
When I bled the brakes several weeks ago, I couldn't get the metering valve shaft to move. Later I started thinking the it was stuck in the "out" position, and last week tried lightly tapping with a hammer, and lo and behold it went in. I drove for the first time, after pushing that shaft, in two nights ago, (mine is normally in, whereas your F250's was probably normally out) and I did notice stronger braking with no noticeable drum brake wobbles from the rear, but it was raining so I was taking it easy on the brakes...

 

Are you absolutely positive you got all the air out of it? That's almost what it sounds like, you're getting a little bit of "pressure tank" effect, and it isn't bleeding back to the master cyl. as fast as it should.

I'd try bleeding it a little more -

 

The brakes are fixed!!

The power booster output rod was adjusted too long thus making the brakes slightly on all of the time. When everything was cold it did fine but I guess after everything warmed up and expanded the brakes started dragging. I guess it was always like this but I never noticed it before replacing the proportioning valve because the fluid leaked so bad out of the valve thus letting the pressure release on the brakes. Thank You everyone for your help and especially ultraranger on suggesting the booster output rod adjustment.

 

I've done a great deal of brake work through 2-wheel and 4-wheel disc brake conversions and such as well as just regular maintenance on them. This isn't the first time I've seen/heard these symptoms and what the problem was causing it.

Brake problems can sometimes be a pain but, glad you finally got it resolved and back functioning again.

 

Yeah, Seen it before. I would have put money on the rod.