My 650G will act like that if I park it 'wrong'. I think the hydraulic pump loses prime. The previous owner told me to always park it looking downhill and it'll never do it. That works. Of course this is not fixing the problem, just bypassing the problem, but I am ok with this rather than spend lots of money to replace pump, whatever. My 650G only has about 2500 hours on it.
A few months ago I parked the 650G 'wrong' on the side of the pond I was building, intending to come back in a couple of hours. That turned into a couple of days. Of course the STOP light came on and no hydraulic pressure. I dragged the front of the dozer to point down hill and restarted it and all was well. BTW, dragging a 20k dozer with the brakes on is not easy.
Bob