Hard Brake Pedal

What is your question?

The pedal on my truck was a little soft until recently when they went extremely hard to push. Occasionally when the brake is pressed the engine will die. If I go a while without pressing the brake, the pedal is soft. The second or third push makes them extremely hard. Brake fluid is not extremely hot. The pads were replaced about two weeks ago. Fluid was really low. Rear reservoir was empty. I did bleed brakes and add fluid. When the pedal is pressed when they are hard the pedal sounds like it is hissing.