Another clutch slippage cause is OVERTIGHTENING of the bolts that go through the YOKES on the CLUTCH LINKAGE.
When we picked up The ARK, the VFD told us that the clutch was slipping. Trying to diagnose the problem was troublesome since it was intermittent and slippage circumstances were inconsistent.
Suddenly one day, while gearing up after a stop at a stop sign, I was about to grab 3rd or 4th gear and the CLUTCH PEDAL dropped to the floor and the CLUTCH would not disengage!
(NOTE: You CAN shift without the clutch, if you know how. You shouldn't JAM first gear without a clutch, but you can restart the engine while in first gear if you've had to make a stop.)
I was only a few blocks and one more stop sign from the house, so I called my wife and quickly explained my MAYDAY! MAYDAY! MAYDAY!... She cleared a "landing zone" at the curb in front of the house for me to coast in to. On approach, I popped the tranny into neutral and coasted in.
Crawling under the truck, the problem was obvious....
Where the Clutch Throw-Out Arm exits the Bell Housing, the Clutch Throw-Out Arm connects to a Rod that has a Yoke on each end. The forward Yoke on that Rod had the bolt at the pivot-point OVERTIGHTENED to the degree that the linkage could not "flex". Over time, the Rod was being bent back and forth like a paperclip.... eventually, the 1/2" STEEL ROD snapped at the aft end of the forward adjustment threads!
NO ROD = NO CLUTCH !
With the overtightened joint, before the catastrophic failure, the linkage would BIND and the clutch would slip.
Replacement of the Rod and properly pinning the pivot point, WITHOUT cinching it down, CURED the Clutch Slippage Problem.
RECOMMENDATION: Inspect linkage for free movement. Assure that linkage is not binding, thus restricting full pressure plate force application for complete clutch engagement. Lubricate linkage as required.
BTW - The Bell Housing weep hole still drips - less and less with time - so I think our tranny over-fill problem is almost cured too.