Front disc brakes lock up

I have a 1990 GMC Vandura 3500 box van (350 engine). For about 6 months now my front calipers will totaly lock up on me while I am driving. I have noticed that if I press the brake pedal about every 5 mintues while driving this will stop them from locking up. I have had new rotors, master cylender, rebuilt rotors. This problem occurs more if I am driving on a freeway. After they lock up of course I have no power and the brake light comes on. If I let it set for about 45 min to an hour the calipers release and my truck will rock forward. After that I can drive again for a few days....or a week without it doing it again. I have taken it to a couple of brake places and they tell me everything checks out OK. They tell me to bring it in when it happens....problem is that by the time I get towed to where ever the brakes are OK by then. Can anyone help!

Welcome to AF. This can happen with a bad brake booster, misadjusted rod to the master cylinder, try disconnecting the vacuum hose to the booster when this happens if the brakes immediately release the booster is bad. In rare cases both brake hose could be collapsing and acting like check valves. Make sure nothing is hold the brake pedal down slightly like a misadjusted brake switch.

Thanks! That does make a lot of sense. I will keep you updated. Thanks again!

Thanks again for your help. I had the booster changed and the problems I had before are gone!
Larry

Good deal, this was an unusual problem though.

In rare cases both brake hose could be collapsing and acting like check valves. Make sure nothing is hold the brake pedal down slightly like a misadjusted brake switch.

Jack, good info but look at the dates, this is over 3 years old and inactive, thanks.