Clutch becomes soft only while engine running

My truck has a ZF5 manual in it. Recently I did the rear main seal and after I installed my transmission and bleed the clutch everything works as it should for a few hundred miles. While driving the clutch became soft. I thought it was trapped air in the master cylinder so I removed it along with the line to bench bleed the system and ultimately replaced it with a new warranty exchange part to try. That did not work. Now the problem is the clutch is bleed until the pedal has acceptable travel and feel and as soon as the engine is started the clutch goes soft with no fluid leakage that I can see on any part of the system and no change in fluid level. I replaced the slave cylinder and that did not solve the problem. I have tried bleeding the system in all different ways, using a vacuum bleeder, pressure bleeding, gravity bleeding and any other way that I found in the book or online. No success.