Brakes Locking Up HELP PLEASE!

Ok my truck is driving me insane. I have tried just about everything I can think the problem started about a month ago when I was on a slight hill the truck wouldn't roll back and over a months time I've replaced almost everything I can think: calipers on both sides, brake master cylinder, rotors, pads, cleaned out the proportioning valve, ect... anyway it seems like the pressure is not being let off of the brakes the strange thing is you don't even have to apply the brakes for the brakes to lock up. Just let the truck run for about a minute or so and you can't do over about 40mph!! any help will be greatly appreciated

I'm not an expert, but check your Vaccum source. Sounds maybe like either its a bad hose/source or maybe if you have a checkvalve in line, it might be bad,

are you certain its the brakes locking up?

Have you jacked up the truck to see which brake is holding?

Have you disconnected the big vacuum line to the booster (WARNING: plug the line, and be aware you will need a LOT more effort to stop the truck) to see if there is pressure building inside?

If you dont have to touch the brakes to get pressure on them, I would start there like Karathos said above.

yeah your booster is cracked ont he inside. replace booster

yea I thought about the booster but I've actually never heard of one going bad, also I thought if one did go bad the only effect would be your brakes not wanting to stop? also i'm sure the brakes are locking up I have jacked the truck up on both sides and they are both locked. also is there a check valve in the system if so where?

also I have taken the line off the booster and there is a strong vaccum on the line so I don't think the problem is there

unplug the line, drive up and down the driveway and see if it will lock up like it usually do

Originally posted by Brandon005



also I have taken the line off the booster and there is a strong vaccum on the line so I don't think the problem is there

I didnt say the problem was in the LINE, rather with the booster. The way you keep the booster from acting is to unplug its vacuum line and plug it.

one time i knew a guy who had the inside of a flex line collapse, u couldnt see it on the outside but it was basically acting like a check valve to the brakes

my truck had to same prob. it was one of the return spring in the drum had broke