Hi Wes, there is not much service information available on the VST/1800 Peerless hydrostatic transmission because Peerless intended that it was not capable of being serviced. It was designed as a throwaway transmission. Here is a simple summary of what can be done however:
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You can get a bit more detail from "Tecumseh Quick Reference and Troubleshooting For Engines and Transmissions", which is available in the manuals section. However note this quote from it:

"���� Tecumseh's current policy on VST and 1800 Series transaxles
with internal failure, is to replace the complete unit. VST and 1800's
have two separate reservoirs which can be checked for diagnostic
purpose only. The output gear reservoir can be checked with a small
pocket rule as outlined in the Tecumseh & Peerless Transmission and
Drive Products Handbook"

Severe loss of drive after warm-up with hydrostatic transmissions usually comes from one of two sources: low oil, or leakage failure past the pistons in either the hydraulic motor or hydraulic pump. No parts are available for the VST/1800 transmissions, so leakage failures are not repairable.

Note that pump and motor wear are traditionally caused by sustained overheating of the hydraulic oil. This happens either by failing to regularly clear debris out of the cooling fins and fan on top of the transmission, or by grossly overloading the transmission, for example by trying to use it as a bulldozer, tow a loaded trailer uphill, or comparable types of torture-test. That is a very light duty transmission, not intended either for heavy work or long life.