Hi, I just bought a 77 F-150 4x4. on the way home both the front brakes locked up. Any idea what could cause both to lock?

 

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Improper pedal adjustment can cause preload the booster causing brakes to drag and eventually lock up.

Damage the booster's internal components by actuating at an improper angle.
Allow slop in the pedal if adjusted too low.

The brake power booseter pushrod that actuates the master cylinder might not be adjusted properly.

Bad brake calipers, make sure they are not frozen and working properly.

An overfilled master cylinder.

- - - - - buck - - - - -

 

Check the condition of brake fluid, if really dirty and grimy, could be rusted up brake caliper.

 

my guess would be the calipers are stuck it happens when ya let these things set to long with out being moved as you were driving everything heated up and exspanded and locked up just a guess tho

 

I agree with the above poster.

Brake components go bad when these trucks sit.

Calipers are cheap to replace though

 

Thanks Guys, I cleaned on caliper and the brakes seem to be working now, but I am working other issues and so am not driving it very far. This truck had been sitting for quite awhile, and the previous owner said he put in a new booster and master cylinder, so if it happens again I'll check that adjustment rod.

 

Well I replaced all the front brake lines, including the center line and put on new calipers. The lines were carefully bled. The brakes still dragged after being applied and released, so I adjusted the rod between the booster and the MC. (It did appear to be adjusted way out). However once I got the rod adjusted so the brakes will release, I find I am pushing the pedal almost all the way to the floor before I get any brakes when the engine is running. With the engine off I get brakes at a more normal hight above the floor. I don't understand why that would be or know what to try next.

I havn't checked the rear brake adjustment yet which could help perhaps, but the distinct difference in pedal travel between engine running and engine off really makes me think it might be more likely a booster or MC problem...