The power brakes on my 1981, F150, 4X4 (81K miles) work fine when they are cold. After driving for 30-60 minutes the left front brake will apply itself, harder and harder, until the truck cannot move. The brake does not apply itself smoothly. When this problem occurs, the truck shakes and lunges, then starts pulling to the left. After the problem occured, I checked all of the wheels for heat. The left front wheel was so hot I could not touch it. The right rear was also hot, but I could touch it for less than a second. The other two wheels were not hot to the touch. The master cylinder and booster were recently rplaced. This sounds like a proportioning valve problem to me, but I would like to get some other opinons/advice before I buy one. Please help. Thanks.

 

[updated:LAST EDITED ON 19-Nov-01 AT 00:48 AM (EST)]It's going to be hard to pin-point it to one certain thing but here are some common things that have happened to me and people on this board.

Caliper pistons sticking, usually after replacing the pads and shoving the corroded pistons back up in the bore with a c-clamp.

Mis-adjustment of the pushrods going to the booster and the master cylinder after replacement.

Strange as it seems, several people have had the rubber lines going to the calipers fail internally, acting like a check valve and not letting the fluid come back.

Oh yeah, these trucks are famous for the rear parking brake cables freezing up from rust. This might be what's wrong with the rear brake.

 

I remember hearing somewhere that the proportioning valve was setup so each side controlled a front wheel and the opposite back wheel. That way in case of a partial failure the vehicle could stop safely. You might try jacking up the front end when the truck is warm and have someone hit the brakes while you spin the wheel that isn't warm to see if brakes are working.

Good luck,
Jeff

 

Thanks for the ideas. Guess I'll start by pulling the left caliper and looking for anything stuck, rusted, crooked, warped, etc. I have a feeling that I am not going to find anything, since the brakes work OK when they are cold. Wish I could think of some way to isolate the problem(s). If I can get the truck home after the next brake locking, I will test the non-locking wheels to see if those brakes are working. I also thought I read something about the brakes working diagnolly in case of a hydraulic failure, but the proportioning valve has only one output port for the rear brakes. There are two input ports, one for the front and one for the rear. There are three output ports, two for the front and one for the rear. At first I thought the brakes were locking diagonally due to the proportioning valve, but now I think it is more likely to be a second problem like a rusted parking brake. This is very complicated.

 

Even though I already had an idea about this :
I just read that an improperly adjusted Wheel bearing can be a culprit for this condition on the front.


Brake

78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles
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Jack the front wheels up and then apply the brake. Get out and turn the front wheels by hand and see if one of them has more drag than the other. If one has more drag, this is probably the one that is sticking. If you find a problem, go ahead and replace both front calipers, they are not expensive only about $12.00 apiece rebuilt.

If you want to experiment with the parking brake cables, find which cable goes to the the one giving you trouble. About an 1/2 inch away from where the cable comes out of the sleeve, clamp some vise grips around the cable. Then take a hammer and beat on the side of the vise grips to drive the cable back up in the sleeve toward the drum. This should relieve it a little if it's stuck and you can see if it helps the heat problem.

 

There is a tiny hole (orfice)in the bottom of each reservoir that relieves line pressure back into the reservoir when your foot is off the pedal. It is possible for the push rod to be adjusted too long and not retract enough to uncover this port when the pedal is released. This will cause fluid to be trapped in the line and it will soon start expanding and applying the brake.If the master cyl cups are retracting behind this port as they are supposed to, then a geyser of fluid will erupt upwards out of the orfice when you first press the pedal. It will usually squirt fluid out of the reservoir (lid off) if you tap the pedal fast.
Another reader mentioned the flex lines closing up and acting like a check valve. This is true. What happens is the hose is restricted and high pressure from the master cyl will push past the restriction and apply the brake but then the fluid is trapped and the brake starts dragging and builds heat which applys the brake even harder. I have disected flex lines with this problem and found the culprit to be the clamp crimped to the middle of the hose that bolts to something to keep the hose from chafing. Over time the hose will close off under this clamp. You can take a screwdriver and open the clamps grip on the hose slightly and solve that problem. A new hose will also solve the problem but the screwdriver approach is quick and easy. You can tell if fluid is trapped by jacking up the front and noting the drag on the wheel, then crack the bleed open and shut it back. If fluid squirts out and the brake releases then the fluid is being trapped.

 

I'll bet its the calipers.Take one off and attempt to c clamp it closed.....Is it difficult to turn in?It should move in nice not hard.If there bad...change your brake hoses too! When front brakes get hydro locked it disrupts entire system because theres no return from pressure to 0.