Clutch Replacement 400D

Reason I ask -- I finally got the tractor running real good, adjusted the TA and clutch as called out by the manual (actually 3 times), and the main clutch started slipping. The previous adjustment by someone else was way out of whack, and when I adjusted it properly, the TA started working but the main clutch started slipping. It slips in all gears but first. When I engage the TA, it stops slipping. I checked adjustments very carefully, and all seems right. The main clutch has free play.

This is my dad"s old tractor, and he tells me the clutch has never been changed, but the TA has been changed once. He farmed with the tractor some 20 years, so the clutch is probably well used.