Very Intermittant Pull to Rt. when applying brakes

[89&94]

I have a 94 Astro that has a VERY Intermittant Brake problem..
A Severe pull to the Right at times when applying the brakes. All the normal front brake parts have been changed Brake Lines, Calipers, Rotors and Pads. My Wife had innitially complained of "Loosing Control" at times when abbruptly applying the brakes, I started out trying to get it to do it for me 1st before changing parts and was unable to get it to act up at all... I went on and started changing parts Brake Lines, Pads and Calipers at first.. She still complained no change.. I then went on and replaced both front Rotors out of desperation! The Problem was still Very Intermittant.. so I took it to our Front end allignment shop and they checked everything took it out and drove it and like me could not get it to pull at all..(Idler Arms were replaced about a year ago)
This past week while I was driving after vehicle had sat for about a week, I was able to get it to Pull VERY HARD to the right when the brakes were applied.. After Firmly Tapping the brake several times, the problem immediately went away and Has not happened in several days. I have spent several hundred $ trying to find this illusive problem and Need Some Advice before wasting any more$. Would it be advisable to Pull a Master Cylinder, Proportional Valve, and or Brake Module off a junk 94 and if so how much can I expect to pay? How much trouble are these parts to change? Any other suggestions would be greatly appreciated!
I NEED a Brake

[Rupesy]

I had same problem with my '93. The reason mine kept pulling was because I got air in the brake line on the driver side. If I braked hard, it would pull, but after a few pumps of the brakes it went away. Almost sounds as though you have air in the lines still or a master cylinder problem. Seems as though I let my brake pads get so low the resovoir for the front brakes went dry. I replaced the master cylinder (needed to anyway) and bled the brakes and haven't had a problem since. Replacing the master cylinder was pretty easy and it wasn't expensive. Worst part is bleeding all the lines.

[old_master]

Guaranteed it's not a master cylinder or proportioning valve. The master cylinder has two lines running from it. One for both front brakes and one for both rear brakes. The proportioning valve regulates pressure to the rear brakes. If you are experiencing a pull in the same direction every time it acts up, the problem is in the opposite side of the direction of pull, in your case, the driver side. As infrequent as you indicated, you will more than likely find a hydraulic problem in the left front circuit. Brake fluid does not compress, air however, does. The first step would be to bleed both front calipers, paying close attention to the driver side for air bubbles while bleeding. If bleeding does not take care of the pull, check the steel line going to the left caliper for kinks, dents, etc. The next step is the flex line to the caliper. Just because it's new doesn't necessarily mean it's good. The inner lining may have a flaw in it restricting fluid flow at times. There also may be debris lodged in a line that must be cleared out. The final component is the caliper itself. It's not very likely; however it is possible for the piston to bind in the bore occasionally. Hope this helps.