My understanding from tearing into them in the past is that the wobble plate is part of the high pressure drive pump. The wobble plate is a disc that has a number of pistons riding round and round on one face. When that wobble plate is flat and perpendicular to those pistons, no fluid goes in and out of the pistons and so no fluid goes to your hydraulic drive motors. This is the neutral position where the tractor doesn't move. When you move your treadle or pedal, it angles that wobble plate. The angle makes those pistons riding around and around on the face effectively move in and out as they go around which pumps fluid in and out. The greater the angle the greater the amount of fluid that goes in and out and the faster your motors move. When you push your reverse treadle or pedal, the wobble plate angles back the opposite direction which reverses flow from your pistons. So it is effectively a variable high pressure pump that goes to your hydraulic drive motors internally in the hydrostat.
Here's a short YouTube video showing visually how it works:
Here's an example torque curve for one of those hydraulic motors. The graph reads torque on the y-axis and rotational speed on the x-axis. You can see that at high RPM the torque drops very low and that there is an optimum torque output somewhere around 2/3-3/4 of the max speed give or take. So this is why backing off a bit when you need more torque makes sense
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There are two independent pumps on a tractor type hydrostat that are both driven by the engine input: the one described above for the drive motor then a second one with a fixed wobble plate angle that provides high pressure fluid to your three-point, steering, auxiliaries, etc.
So from a less theoretical and more practical standpoint, it is possible to have one pump go bad without the other; as it is possible to have a linkage problem. The linkages can be lever or cable type but they are complicated mechanical assemblies. I would start by taking off whatever covers you need to to visualize that linkage assembly and with the tractor off manually actuate your treadle or pedals and look closely at that assembly.
If that assembly is functioning like it's supposed to, then start checking pressures with an oem manual as suggested above