I have an 86 GXL and I do not have ABS.

When I drive for awhile usually on highway the brakes start dragging and the pedal gets tighter, to the point where I barely push it and the brakes are applied - well the other day I got out and took the master cylinder cap off and pumped the brakes then started the car and left again and the pedal was back to normal. Is there some kind of pressure building up that is released when I take off the cap?

 

there is no pressure between the cap and the fluid, taking that cap off will not do anything. unless your brake pedal gets soft or your breaks begin to fade after short stops is when u shoudl get worried seems that everythign is fine with yours..