There is a lot of debate surrounding the risks of overfilling a hydrostatic transmission, so I'm sharing my experience with this (already solved) on a Kubota B7800.
Despite removing every single drain plug, both filters, and the strainer, 3.5 gallons (13.2 liters) of new fluid put me over the top mark on the dip stick by more than an inch. The manual lists the fluid capacity as 15.3 quarts (14.5 liters).
I let it idle for a while, worked the steering lock-to-lock, then all of the FEL and 3PH controls to work any air out, but the indicated level remained more than an inch above the top mark on the stick. I thought maybe it would take more time operating the tractor for the fluid to settle in, so I drove back to the other side of my property and started using a finish mower.
After mowing at proper engine RPM for about five minutes (mid-gear but with the pedal only halfway down) the tractor seemed to be slower than usual, as if the brakes were engaged. I bumped the transmission down to low gear and that also seemed unusually slow. Then I tried high-gear and that appeared to work just fine, accelerating normally and reaching the typical top speed. In each of the gear-range tests I tried with and without the PTO engaged, which did not seem to have an effect.
The overfilled HST was the obvious next thing to address, so I siphoned out fluid until the level was correct. I removed approximately 1.5 quarts, which brought me down to the top ridge of the OK range on the dip stick. The siphoned fluid appeared to be full of micro-bubbles, which seemed like evidence of a potential issue.
After letting the tractor sit for a few hours with the proper fluid level, I restarted it and spent a few hours mowing without experiencing any loss of driving speed in the low, mid or high range. The problem was apparently solved.
Hydraulics is not my particular area of expertise, so I can't say for sure that overfilling was the core issue, especially since the fluid had just been drained and some lingering air might have remained in the system until I operated the tractor at high RPM with a PTO load.
For anyone who is changing the fluid in a B7800 or equivalent, be aware that the system might enter the "acceptable" range with less than 12 quarts of new fluid. In my case, I started with 14 quarts and removed 1.5 quarts, so ultimately it took 12.5 quarts of new fluid to hit the target.
Despite removing every single drain plug, both filters, and the strainer, 3.5 gallons (13.2 liters) of new fluid put me over the top mark on the dip stick by more than an inch. The manual lists the fluid capacity as 15.3 quarts (14.5 liters).
I let it idle for a while, worked the steering lock-to-lock, then all of the FEL and 3PH controls to work any air out, but the indicated level remained more than an inch above the top mark on the stick. I thought maybe it would take more time operating the tractor for the fluid to settle in, so I drove back to the other side of my property and started using a finish mower.
After mowing at proper engine RPM for about five minutes (mid-gear but with the pedal only halfway down) the tractor seemed to be slower than usual, as if the brakes were engaged. I bumped the transmission down to low gear and that also seemed unusually slow. Then I tried high-gear and that appeared to work just fine, accelerating normally and reaching the typical top speed. In each of the gear-range tests I tried with and without the PTO engaged, which did not seem to have an effect.
The overfilled HST was the obvious next thing to address, so I siphoned out fluid until the level was correct. I removed approximately 1.5 quarts, which brought me down to the top ridge of the OK range on the dip stick. The siphoned fluid appeared to be full of micro-bubbles, which seemed like evidence of a potential issue.
After letting the tractor sit for a few hours with the proper fluid level, I restarted it and spent a few hours mowing without experiencing any loss of driving speed in the low, mid or high range. The problem was apparently solved.
Hydraulics is not my particular area of expertise, so I can't say for sure that overfilling was the core issue, especially since the fluid had just been drained and some lingering air might have remained in the system until I operated the tractor at high RPM with a PTO load.
For anyone who is changing the fluid in a B7800 or equivalent, be aware that the system might enter the "acceptable" range with less than 12 quarts of new fluid. In my case, I started with 14 quarts and removed 1.5 quarts, so ultimately it took 12.5 quarts of new fluid to hit the target.
