Sometimes my brakes on my 1988 truck will not release. I have to pull off the road & remove the vacuum line clamp on the Power Booster & vent the line to get the brakes to release. There is a plastic looking valve (Pt#44730) halfway between the booster & the vacuum source on the vacuum line, could that be the problem ?

 

Why don't you start with the factory-prescribed operational checks, and go from there? http://personal.utulsa.edu/~nathan-b.../3checksan.pdf

 

I would strongly suggest making sure that your booster is not going bad. Test them by with the engine off pumping down the brake pedel until it gets hard, then start it while holding the pedal down you should feel the pedal drop slightly if it stays hard your booster is bad or you have a very bad vacuum leak. A vacuum leak at the booster line would be obvious the engine would not run very good. I have had to open a bleeder valve on some to release the brakes. If you had brake work done recently I would do all the checks from the suggested web sight.