Late D7F's, with the small transmission (like a D6c) have an "o" ring on the suction line at the bottom of the transmission housing, where it bolts onto the backend. They can suck air here if the "o" ring is not good. Have seen mechanic's leave this out causing the same problem you have. Only way to check this is pull the trans out. D7F's can also break up their brake linning's and the pieces get sucked into the suction line. If it has the later trans there is a suction passage in the back housing, under the bevel gear, leading to where this "o" ring is. The passage is an odd shape and it is hard to see if there is anything in it. I have see brake linning jammed in here causing a suction problem. Transmission screen is near the trans pump on the back right hand side of the motor and should be checked first, also check the "o"rings on the top and bottom of this screen, and also all the "o" rings on the suction hose from the back housing to this screen. Could also be a worn pump but does sound like a suction leak. D7F's also have a tendency to lose their transmission pressure when working on steeper slopes, Caterpillar's recomendation to fix this is to run your transmission overfull by 2 gallons. Good luck 68M