Small Engines (Lawn Mowers, etc.): Loss of hydrostatic transmission power, wheel mode, clutch pedal


Question
I have an Ariens Sierra 1540H mower (Model # 936026, SN 000850) with a Hydro-gear 325-0750 transmission that recently lost power to climb hills even though there is no loss of engine or cutting power or cutting power.  Some power continues to be transmitted to the wheels so that the mower will travel on a level surface, but it will no longer climb even a gradual grade.

The drive belt appears to be OK.  After reviewing an online recommendation, I drained the unit and refilled it with new 20W-50 oil as indicated by the mower service manual.  That made no difference in the mower's operation, so my next repair attempt was to remove the transmission from the mower, open it, and inspect it's major components.  They all appear to be in perfect condition.  I replaced the filter, reassembled the unit according to the service manual directions, and replaced it in the mower.  Again, there was no improvement in the performance.

Other than the transmission, the mower is in top condition.  Any recommendations you have would be greatly appreciated.

Answer
Some of the hydro parts may just be wore causing it to lose pressure.  Did you purge the trans after filling oil?

Generic purging procedure.
Place the "free wheeling" lever into free wheel mode (transmission will not drive this way).
Sit in seat and start engine with brake/clutch pedal down, and the trans lever in neutral.
With engine at idle RPM, let the brake/clutch pedal up.
Move the trans control lever to FULL FORWARD and hold it there for a minimum of 5 seconds.
Move the trans lever to FULL REVERSE and hold it there for a minimum of 5 seconds.
Repeat the Full F to Full R at least 3 times.
Stop the engine and move the free wheel control to the engaged position, make certain it fully engages.

Did you fill with 81 oz. of oil?  

Eric