2000 Buick Lesabre brakes won't build pressure

Hello. I just replaced my rear brake line and rear calipers. I bled the system and it built up proper pressure, but when I turn the engine on it losses pressure. Just going up and down the driveway the car seems to stop OK but it still take a lot of work, as if the lines were not bled.
As a note, when removing the rear lines, I was attempting to heat a coupling with a torch and did accidentally catch some of the insulation on the firewall on fire. I put it out quickly. Upon inspecting the area it appeared the only damage was to a wire that I believe is for the ABS system. While I will get that replaced I don't see why it should adversely affect the normal performance of the brakes.
Is it possible the fire damaged something the brakes need to operate? I have bled the system from the rear before and have never had this issue.