I just picked up a 91 f-150 xlt lariat , 4.9L, 5 speed. Guy who was previous owner passed away, son sold me the truck. Said his dad replaced the clutch himself right before he passed, but never worked right. The clutch is slipping pretty bad, but made it the 7 miles to my house. Without pulling everything apart ( which I will probably do anyway ) what should I look for as to why it would be slipping?

 

Glazed flywheel, grease or oil on the clutch or flywheel, possibly the throw out bearing not seated properly or not relaxing like it should keeping the pressure plate slightly engaged.

 

There is an inspection plug on the side of the bellhousing.