Brakes lock up once the 280 1998 is driven to work and halfway home 25 miles to work. If I open the stem valve on the rear brake part of the master cylinder and let out a squirt of brake fluid they work just fine. I have the brake as far off the floor as it will let it go.

Sorry 1976 280

Sounds like you have the pedal adjusted too tight and it's keeping the pistons in the master cylinder from retracting all the way. The fluid gets hot and expands, putting pressure on the brake lines, cylinders and pistons, because the fluid can't get back in to the reservoir.

Did you adjust the brake pedal recently? Loosen the rod going in to the master cylinder until there is some play between it and the brake pedal.

Where is the adjustment done? Does anyone have any pictures?

Under the dash at the end of the brake pedal. If you have flexible fingers, you can do it without removing anything, just loosening the lock nut. Lots of good pictures and a description of the procedure in the FSM, Brakes chapter.

http://www.xenons30.com/reference.html

If no one has messed with the brakes recently, it could be something else. More details about the recent history of the car would help. Good luck.

Two other things that could cause the same problem are: the adjustment between the MasterVac and the master cylinder and the little bleed hole in the master cylinder is blocked.

When checking the pedal free play you should be able to detect 2 distinct points where free play is taken up. The first is the pedal to MasterVac free play; the second is the MasterVac to master cylinder free play.

Rear brake cylinders can corrode inside and cause this.