I am having a problem with the brakes on my 1990 Silverado. As I drive the brakes get tighter and tighter until they are totally locked up. If I undo the brake line at the master cylinder and let some air in the line (which I know is not the best idea). Then the brakes become soft enough to allow the truck to be drivable again. When I do this the brake start off extremely soft but within 10 mins of driving they become tighter and tighter until it they are totally locked up again. Any suggestions about what might be causing this problem would be great.

 

Sounds like you need a new caliper, or bleed the lines.

 

Brake Combination valve under the master cylinder bolted to the frame.

 

The first thing I would do is give it a brake fluid flush to make sure you had the right fluid in it. Also make sure the brake pedal has a little free travel in it to assure the M Cyl is releasing all the way.