If anyone could provide assistance. I have a 1989 SR5 pickup and the brakes seem to lock-up after a few miles of driving. I took it to a mechanic last week and he said that he could not find anything wrong and that the truck ran great. I drove it around for a while and today they locked up again.

Today I finally checked the fluid level and it seems low. I will add more fluid and I am making an assumption that the fluid level being low is causing the float in the cylinder to drop and seal around the opening thus creating a vacuum.

Could this happen and would this cause the brakes to lock-up?

 

Master cylinder is NG.

 

Are all of the breaks locking up, or specific ones?

 

To test, disconnect the line for the front brakes (if that is your problem) from your master cylinder. Oil should continue to drip out of the master until empty. If it does not drip then it means that the lines are not leaking back to the resivoir thus "pumping up" occures due to the pads bouncing off the disk while driving.