My brakes are soft and then when you pump them once they lock up what could be the problem

 

Does your vehicle have a gas or diesel engine?

 

It is a diesel

 

Was any work done to the brake system just before this problem showed up?

 

That's the problem no work has been done and it just started I replaced the brakes 2 years ago and that the last time they were touched

 

To possibly take the ABS out of the equation ,try removing the ABS fuse and see if there is any difference. If yes, then there is a problem with the ABS section. If not, here could be a crushed line or the hydroboost unit has a problem.