Per Thetford:
) Harder than normal pedal or hand lever operation:
Apply light film of Thetford Toilet Seal Lubricant & Conditioner or silicone spray to blade/ball.
(Note: To avoid damage do not use spray lubricants other than silicone.I have cleaned the seal/ball lubed it with plumbers grease and foot pedal still would hang up.
Finally disassembled foot pedal and cleaned/lubed 'waste ball drive arm' (#10) and the 'slot' it fits into on toilet/
Re-assembled (PITA due to large spring in foot pedal) and it slammed shut for about a month (Full Timers use...!!)
Then started hanging up again.
Next destination..........RV Store and new Dometic 320 w/sprayer
LINK