1992 GMC Jimmy Brake pedal gets soft and light comes on.

Last fall, my brake pedal went to the floor and my abs light came on as well as the brakelight indicator. My local mechanic told me I needed a new ABS pump. I opted for a used one off of ebay. When I got the car back, all was well. Both lights were off and brakes seemed to work fine until I was coming down a long hill and just rested my foot lightly on the brake pedal. The pedal slowly went down and nearly bottomed out. The brake light also came on. (Not the abs light) One pump of the brakes and everything was fine. If I jump on the brakes or anything else, the brakes work fine. I can also hear the ABS "hum" so I think that is working. But if I slowly depress the pedal like in stop and go traffic or a long slowing down a hill. The pedal goes down and the brake light comes on. I have had it back to my mechanic 3 times and he has told me that it can be very hard to bleed those abs pumps. A special scanner is needed? He borrowed one from another mechanic this last time and it still is not right. I spoke with a GMC mechanic who told me how to bleed it manually and I am confident that there is no air in the abs pump or any of the lines, yet the problem continues. I am left with the options of buying another used pump or having this one rebuilt for $500. Or could there be another problem all together? Any and all help would be appreciated!