My experience was that the brake pressure compensating valve needed a huge amount of pedal pressure to get it to start flowing, then it worked great. I had the same experience with the pressure bleeder, they don't work on the newer master cylinder with the floats.I also couln't get the rear brakes to flow with brake fluid. I just finally ended up pushing the brake pedal really hard and it just went down all of the sudden and everything worked fine. I had taken the pressure compensating valve apart to clean it up. The valve is very simple, just a spring piston. I think it gets in a relaxed position and you need to push the piston back to get the fluid flowing to the rear brakes. I did bench bleed the master cylinder in a vice first, but it may not be needed.