I replaced the brake booster and master cylinder on my gs this week due to the brakes feeling spongy and slight dampness at base of master cyl next to the booster i had bench bled the master and had a hard time getting fluid to the rear wheel cylinders i got another new master cylinder and bench bled it also still no fluid to rear.i tried to bleed by pumping brake pedal into a line in a bottle of fluid. No luck had neighbor pump and hold while cracking the bleeder screws no luck. I tried vacuum bleeding no luck. I noticed about 1/8 inch tension on the master cylinder when tightening the nuts so I loosened the nuts and let the tension off and it bled fluid to rear normally as it should . All 4 wheels are bled now and feel decent but I haven't tightened master cyl nuts yet . I just found the thread about the button on the proportioning valve I haven't pushed it yet. The pedal drops slightly after you hold it down for 15 to 20 seconds. Maybe I'm losing vacuum at the booster master union because nuts arent tight. I measured old booster and adjusted new one to same length at clevis on pedal. I may need to shorten the clevis a bit and tighten the nuts I feel fairly good about the bleeding of the brakes. I am wondering if I should rebleed with proportioning button pushed?