soft pedal after power disc swap

[skyjgmc]

Hi Ive asked this question over in my build thread but getting no response so I figured tech is where to ask. link to my build threadhttp://www.67-72chevytrucks.com/vboa...d.php?t=689194
I recently aquired a 1970 gmc 1/2 ton truck it had manual drum brakes on it I drove it like that for 2 months with no issues but was getting tired of the hard pedal. I had a dana 44 front axle out of a wrecked 1985 jeep J10 truck so I robbed its front spindles and rotors and swapped them on along with new gm 1/2 ton calipers with new 1 ton rubber hoses and ran new hard lines to them. swapped in a power brake booster bracket out of a junk yard along with a brand new booster/master cylinder. got a proportioning valve out of a 74 gm truck with disc/drum at the junk yard. I got everything hooked up gravity bled the system them bled with a person pumping/holding the brake while the other loosened the bleeders.
It stops ok with the new setup and now after Ive let it sit then start it up and take it for a drive the first time the brakes are applied I get softy/spongy pedal it still stops just with longer pedal travel. then the rest of the drive the brakes work as they should with no soft pedal. if I dynamite the brakes the backs lock up but not the fronts they work so so ( just not as good as I expected) this is baffeling to me is the proportioniong valve doing its job or getting stuck with only the back brakes working, its the only thing in the system thats not new. I bench bled the master real well before I installed it. Ive done lots of searching and all Im finding are others who have had simliar problems seemed to get soft pedal more often. Mine does it once during the drive and thats it. In the research Ive done others have adjusted the rear brakes and seemed to have fixed the problem while others have lenghtened the push rod from the pedal to the booster for more pedal travel or just redrilled the hole 1" lower. just lost on where to start? here is a pic of ol' Walter for motivation