1995 Acura Integra Brakes mushey when hot.

When I first start the car the brakes are solid. After driving for maybe a half an hour depending on how many starts and stops and the outside air temperature the brakes get mushy. The pedal's initial travel will be more and the pedal will drift down from there. The pedal doesn't reach the floor but it's way too low. Pumping it brings it back up. This all started after I had some work done that involved replacing part of one brake line that ran to a rear brake. I'm not loosing any brake fluid and I've bleed both rear brakes but the problem has come back. I'd rather not bring the car back since the work was some of the sloppiest work I've ever seen and I've brought it back once already to fix other problems with the work that was done.