I think your 755 is similar to my 770 in that it has both a power steering and an implement hydraulic pump. So the chances of both failing simutaneously is slim. And with any mechanically problem, look at the simple and cheap options first. My guess is that the pumps, which I think use a common suction, are restricted due to a blockage on the suction side. Not sure about the 755, but the 770 and 790 have a suction screen (often found clogged with "stuff") as well as a spin on filter. To access the screen, you must drain the transmission/rear axle/common sump.
I'm pretty sure that if you drain the fluid, clean the screen, replace the spin on filter and replace the fluid with new clean tractor fluid (the JD stuff is good and fairly reasonable) you will find that the machine is back to normal. If the HST has a filter or screen, it should also be cleaned or replaced.
