Heres an interesting problem i just encountered in the last two days that i thought i would share w/ eveyone in the hopes it would help someone out while bleeding front power brakes. While bleeding my brakes on my 69 z28 the rear bled out fine, however the front would not bleed. i continued to get massive air from my bleeders. I removed master cyl. and bench bled it thinking there was air trapped in master. There was a small amount but not like what i was getting on the car. To make a long story short, the problem was the round proportioning/ accumulator valve under the master cyl. New air was being introduced into the system from the plunger on the backside of this valve. I hooked this valve up to master cyl. in vice on bench and worked the master cyl. plunger. Put a drop of brake fluid around plunger on valve and watched it suck in the brake fluid when master cyl. was released. Air was sucked in every time brakes were let off, yet there was no brake fluid leak at all. Ordered a rebuild kit from heartbeat city hopefully this will take care of it. Please respond if any one else has seen this problem.