Please help, Front Brakes are locking up

[Black1987Stang]

Im thinking it might be the proportioning valve or maybe the master cylinder....this is how it happens, brakes are all fine for normal braking applications, but once you press them harder say for slowing down harder than normal the front brakes LOCK UP, if u drive around (well move forward slowly lol) for a little bit it will free up and be normal again......BTW its on a 79 Bronco with a 351 Modified....I no I no its not a mustang but at least its a Ford with a good motor .....What could it be? :SNSign:

 

[dragnazz5.0]

are the back brakes adjusted properly? if they are than its probably a metering valve problem or a proportioning valve problem. more than likely its happening because your rear brakes either arent working properly or the metering valve isnt keeping pressure off the front brakes until a certain amount of pressure builds up in the rear brakes....thats what im thinking at any rate

 

[Black1987Stang]

I gotcha where is the metering valve located?

 

[HISSIN50]

It can also be a damaged caliper (piston/bore relationship or a piston seal issue).

Good luck.

 

[dragnazz5.0]

It can also be a damaged caliper (piston/bore relationship or a piston seal issue).

Good luck.

i would think its more of a master cylinder issue than a caliper issue. maybe the master cylinder is bypassing at a short travel and than when you really hit the brakes it applies full pressure. if it was a caliper i feel it would just drag all the time and not lock the brakes up and than get better. but it could be a caliper issue if the piston is seizing up and than breaking loose.

to the original poster....the metering valve should be in the lines right off the master cylinder.....i know on my car its the same unit as the proportioning valve

 

[jrichker]

On cars with old rubber brake lines, the lines will seperate inside and cause a one way check valve effect. You step on the brakes hard and the fluid gets trapped inside the line and does not return to the M/C like it should. Since the check valve effect isn't perfect, the lines will loose pressure and gradually return to normal. Envision it like a balloon that swells up and pinches the center part of the line closed.

 

[bt15]

Jrichker's description is not just something you can picture in your mind's eye. My old 88 Bronco began to exhibit similar symptoms to your truck, and when I crawled down for the old "visual inspection," one of the brake lines actually had a clearly visible pregnancy on it. I replaced both lines just to be safe - problem solved.

A bad master cylinder and a hanging caliper each manifested similarly on my truck, so take a peek. Man, that Bronco of mine ATE brakes - I was always working on them, repacing pads, calipers, lines, master cylinder, slave cylinders, etc.

 

[Prime Lord]

Sounds like a bad line or a bad caliper to me. If it was the M/C, it would drop w/o any brake pressure.

 

[tbovee]

I too would have to go with the hanging caliper or bad hose. I see it all of the time at the shop.