Rear brakes question

[Dannyboy1966]

My 86 GMC is doing something I have never seen before on a vehicle. I recently went through the brakes on the rear, new steel lines on the axle, new wheel cylinders, brakes, drums, and parking brake cable on the driver side. Now mind you the truck isn't sitting on a hill, but when we cracked the bleeder screws open, for some reason they bleed themselves. You can walk up to the master cylinder, and watch the fluid go down, and come out the bleeder screws. Is there something wrong with my master cylinder or something/ I've never seen this before. It has the hyrda-boosted brakes on it. Seems to stop fine, was just wondering.