I just recently bought and rebuilt truck, and the trannismission fluid keeps it seems just dissapering, nothing is leaking when parked and everything is dryunder neath of truck, any suggestion of whats happening?

 

Check the cooling system for transmission fluid, its very common for the transmission cooler in the radiator to start leaking letting the fluids mix and causing major problems.

 

The vacuum sensor on the side (load sense) may have a broken diaphram and the engine is drinking tranny fluid through the straw. If your radiator is not overflowing with red oil, its going out the tailpipe.