You mentioned that the shop replaced a caliper which would require them to bleed all four brakes. Often the bleed valves on the rear brakes get corroded and they cannot be opened to bleed or they break off when you try to open them. If they were opened to bleed them you should be able to open them now. If they are frozen shut or broken off, good bet that the shop did not bleed the rear brakes and you have air in the lines. As I mentioned earlier, the problem you described sounds like the breaks need bleeding. It could be other problems but I would try getting a good bleed of the brakes first.

You may have to change the rear brake cylinders if you cannot get the bleed screw loose. If you can get the brake line disconnected, it is not a hard job to replace and not very expensive. (about $20 each).