I have been performing DIY small equipment repair and maintenance for many years, but have no experience with hydro-static drive transaxels until now. I recently picked up a Craftsman ZT7000 with a Kohler 20HP drive. The engine runs great, but I’m experiencing serious and uncontrollable lurching when trying to move forward or backwards. I have read many threads that have lead me to check for a missing/broken spring and to check the oil levels in the transmissions. I have modified both transmissions with 1/8” drains and drilled the tin to get access to the top oil fill plug, and drained and changed the oil with recommended 10W50 Diesel grade engine oil. I have adjusted the coil spring tension on the brake control shaft between the two transmissions to different tensions within the range of adjustment, but none of the work has corrected the lurching. The drive belt tension seems adequate so I don’t suspect drive belt slippage and the drive belt is new. Anyone have any direction for me before I condemn the transmissions?