Problems after power disc brake conversion ??'s

[casterbuster]

i recently completed my ece power disc brake conversion. In doing so I replaced all of my brake lines as well. I also replaced my rear emergency cables.

I got this all finished up Sunday morning, everything was working great.

I had to leave the truck at my dads, I live 3 hrs away. He called me last night he said he had been using the truck to move some stuff around in the yard. He then said he was doing quite a bit of backing up. Now, he says the rear drum brakes are locking up. He said that he can loosen the line fitting on the proportional valve to bleed off pressure and the brakes will release, but will start to lock up again shortly afterward.

Do you guys have any clue what might cause this?