brake trouble

I have a 72 Scamp with stock 9" drums. I have a bit of a problem. A while back I replaced all 4 wheel cylinders and removed my master for replacement. I thought it was a good idea to clean out the lines so I blew 120psi of air through them for a clean out. I connected the wheel cylinders and installed a new master. I filled the ports for a gravity bleed and nothing is going to the rear. I was doing some reading and figured that when I introduced the air, the proportioning slammed shut to the rear. Is there any way that I can reset it or will I have to replace it?
Thanks,
Steve