2000, S70 GLT SE, Brake Issue

[Rbaratt]

I have what appeared to me to be similar to a master cylinder going bad. The pedal travel would sometimes change and be lower to the floor before I feel the brakes start working. Pump a bit and they grab where they normally do. The issue was intermittent, but eventually it became constant. The system was inspected and there were no leaks. I retrieved a donor master cylinder from the junk yard and installed it. After bleeding the entire system at all four wheels I still have the same issue. I continued to drive and yesterday the brakes were back to normal, not sure why but it is probably back to being intermittent again. Is this issue related to ABS control module, modulator or pump? Any advice is greatly appreciated. Thanks in advance.