Haven't worked on a tractor HST (yet), but I've done a few riding lawn mower HSTs. In those, the HST has two adjustable plates, one in the transmission pump (engine side) and another on the transmission motor (axle side). One plate is adjusted by the forward/reverse pedal, the other plate is adjusted by range selector. This F/R pedal causes the fluid to change direction of flow. Often these designs will utilize two different overflow values, one for forward and one for reverse. Because of this, many owners will notice a loss of power in one direction, while things seem normal in the other direction. Most often the forward directional value is the first to start failing. I've only seen a handful of cases where the reverse value started failing before, or at the same time, as forward. All those belonged to owners that were from areas with generally very hilly land, which may had contributed to the equalization in failure rates
Usually (90% of the time) most failing valves will fail a 'spring length test' - where you simply measure the springs length (no compression), and if its not long enough, it fails. Only a few failing springs will pass the above test, and have to be compression tested to detect the failure.
Typically, if someone comes it with such a problem, and their machine has 500+ hours on it (less if they have very hilly lots - I ask). I won't even bother to check the springs and just replace all the values outright. Sense you have it apart anyway, right?! An entire set of values would likely be $200-300 (guessing, based on cost for the small ones I use) for your tractor, plus whatever labor is. I always replace them all at once - especially if one had a detectable failure. As it saves the guy another trip to the shop, 50-100 hours later, when the other direction starts failing.
You may want to check if a rebuild kit is available. These would normally include a complete set of overflow/bypass values, d-balls and/or needles, return springs and most importantly a full set of gaskets.