Camaro4LifeSS

I was finishing up my ABS delete tonight and ran into a problem while bleeding the brake lines.I could'nt get any pressure to the rear lines.It has plenty of pressure at the front but none at the rear.I took the rear line off at the master cylinder to see if it had pressure out of the MC and nothing.I took a air hose and blew a little air in the MC and bam I had some pressure.Went to the rear and the first time I tried bleeding it bled fine.Tried it again and no pressure once again.I wonder if theirs some trash in their or something.I have another MC laying around that i'm going to put on tommorow that i'm hopeing will fix it.Is their something that I may have missed that could be causing this?I also need to add that I had the MC off while I was doing all this because I painted my engine bay but I plugged all the holes.
Thanks,
JE