I have one of the few Scags Magnum Diesel grass cutters.
I have been experiencing  a problem that maybe someone might know about . 
HST works good, but it wants to pump when I change directions.  Pump like the whole tractor bounces.  I can relieve the problem by cutting back on the RPM's or pampering the foot Hydralic pedal. 
Only happens when I change directions on the cutter.  Fluid is where it should be.