Case 455C Dozer/Loader

I have a Case 455c which sometimes shows zero transmission pressure on the gage just after starting it, the machine will not move. What is weird is that all of a sudden the pressure will come up and I can work the machine for hours, the next time I shut it off the same thing repeats. I replaced the filters, made sure the oil was up to full, and am not seeing any leaks..
Would and of you Case guys out there have an idea of where to look for correcting the problem.

Thank you
NewDozer:(

Case 455C Dozer/Loader