1976 F-150 C.S.

I have a leak at the input shaft seal, It leaks like crazy after I drive it but doesnt leak at all when in use.
I have sent it in and had a new gear put on it and this started to happen. Any ideas on why it leaks after being parked and leaks nothing when driving? I can only guess pressure is pushing on seal while in use causing a seal then leaking when cool?

 

Assuming your C/S has P/S:

 

Yes, it does have PS It seems odd it would leak up hill after the truck is shut off over a period of time, the longer it sits the more it leaks. I know it wants to be driven but this is ridiculous

 

After reading in the forums about problems with using power steering fluid instead of Type F in a Ford PS unit. I realized I was using normal PS fluid. Once I switched over to type F the steering shaft leak quit almost immediately.