Front brakes apply by themselves

[79bronc]

I am new to this but here goes. I have a 1979 Ford Bronco with original axles. I put a new master cylinder probably Feb 2010 because it was leaking between master cylinder and booster. I have also replaced front brake lines. In June 2010 I had both front calipers start sticking one day and then it didn't do it anymore until Nov 2010 but I also don't probably put 150 miles on it a month. But the thing is sometimes it sticks after you press them then sometimes you can be driving and not press brakes for 4 or 5 miles and all of a sudden both front brakes will start locking down without ever applying them. The interesting thing is you can get out and loosen the front bleeder screws and fluid will shoot out about 8 or 10 inches and they will release and you can drive again. I replaced both front calipers about 2 weeks ago and still doing same thing. The right one is worse than the left but both will do it. I drove it maybe 10 miles today and had to stop and bleed the pressure off about 4 or 5 times. Any help would be appreciated this thing has me stumped. I have seen vehicles brakes stick and not release but never start locking down without applying the pedal. Thanks in advance.

 

[Bronc76]

You said you replaced the lines, did you also replace the hoses that attaches to each calipher? Just finished a brake job where the brakes were sticking because the hoses were collapsing because they were dryrotted.

 

[Bully Bob]

Naw.., that sounds like the push rod between the pedal & the M/C

It is adjustable & probably needs to be shortened to allow fluid pressure to release... /emoticons/[email protected] 2x" width="20" height="20" />

 

[Seabronc]

I had a similar situation but only on one wheel. It turned out to be old rubber brake lines that had separated inside and acted like a valve to prevent the pressure from releasing from the brake cylinder.

Good luck,

>-

 

[69bronc347]

i have ran into this many times, and it has been a variety of things, has been the rubber lines, has been the push rod miss adjusted, but most of the time it has been the brake booster. heres a way you can tell, start with driving until the brake get stuck, the easiest way to diagnose this problem is to disconnect the vacuum line to the brake booster, if the brakes unstick its the booster. if that dont work take a 1/2 inch wrench and slightly loosen the master cylinder from the booster if the brakes unstick the pushrod needs to be adjusted, if that dont work, more than likely its the brake lines. both the front brakes its probably the line between the frame and the diff, this however does not rule out a mechanical problem with the master cylinder

 

[BUCKIN'94WFO]

It would be interesting to know if the brakes do their lockin' up thing after criusin' at speed then abruptly lettin' off of the throttle. Quick application of manifold vacuum could play with a booster that's goin' bad. I've seen brake lines act like what Seabronc was sayin', only different. It was an old 67 Chevy 4 wheel drummer that ended up havin' brake lines that were blistered internally. You could STAND on the brakes and they barely apply. Turns out all the hydrualic force was just makin' the rubber blow up in the inside of the lines shuttin' down flow the wheel cylinders. Drove me bonkers. Once I figured it out though I had a customer for life!

 

[Marshel]

I am facing the same problem and even thinking to replace both front calipers. your post is useful for me, now I am thinking to do the same with my Ford Bronco.