1989 GMC C2500 slow clutch return

Drive Train Axles Bearings problem
1989 GMC C2500 V8 Two Wheel Drive Manual unknow miles

The clutch on this truck has good pressure, disengages properly when depressed, has no leaks in the master or slave cylinders [fluid level in the reservoir doesn't go down], but when the clutch pedal is quickly released there is a lag of 1 to 3 seconds before the clutch engages. This truck had sat without running for approx. 1 year before being put back on the road. The clutch operated fine for a couple of days before this problem suddenly developed. I then noticed that fluid in the reservoir was almost black so I bled/flushed the entire system but no change; good pressure and release, slow return. I, m now suspecting swelling of the rubber section of the hydraulic line. Any advice on this?