Brake fluid flush, what do you do?

10 psi RPV's just 'check valve' hold some back pressure to maintain the cup to wall integrity. If you pump the brakes, you set the 10 and you should be able to crack the bleeder and get a little squirt, but its a check valve to prevent a back flow. Once the bleeder is cracked the pressure is off the RPV , it will relax and gravity will pull fluid forward through that toward the brake cylinder. I believe this is how those work. It takes 75 PSI to overcome the drum brake return springs!