Nissan Repair: brakes pulling, pressure control valve, nissan pickup

Question
I have a 1993 Nissan pickup with 146000 miles. Lately the brakes have had a very noticable pull. 90% of the time the brakes pull hard to the left, but every once in awhile when I step on the pedal really hard, they pull to the right.I have had 2 mechanics look at the brakes with me & they can't see anything wrong. The pads & rotors look fine. The brake fluid pressure in the brake lines is equal(although I didn't personally see this with the mechanic). I had the calipers & master cylinder replaced & this did nothing to help. What could be wrong? Is there any kind of brake fluid pressure control valve that isn't part of the master cylinder?  

P.S. Also, when the master cylinder was put on  & after the brake lines had been disconnected, the brakes suddenly started pulling hard right for awhile . Then the pulling gradually went back to the left predominately.So I feel that the problem must be in the brakes somewhere & not in the suspension or steering.  

Answer
Tim,
Well when they replaced the master cylinder they should have bleed it.  This only thing I can think of is that you have to much pressure built up on one side.  

You may have to go back and redo the system AFTER you flush it (brake fluid).  Then just fill the fluid back up and go thru the process of bleeding all four wheels.  

See if this will solve your problem.  This is the only thing I can think of is that you have to MUCH PRESSURE built up on one side.

Hey, when you take your hands OFF hte steering wheel does is drift to the right.  

If so this could be your alignment.  So when you brake this will cause the drifting to the right.
Darrel