Here's the upshot on the John Deere 455 with the non-func tranny.
Check out the hydraulic schematic on
The prior "mechanic" had welded shut the actuator pins on both "tow" valves. For good measure, he brazed the balls in their seats.
The tranny had low-power forward, but no reverse. Since the same motor and pump provide both directions...
Look at the drawing, and you'll see that the hydrostatic gets its fluid charge through the two valves he welded shut. It can suck some fluid through the "anti-cavitation valves", but would never be able to fully purge the circuit of air unless the charge is provided under pressure -- which it could not be with the tow valves soldered shut.
With some small trepidation, I ordered new valves - $353 worth - and screwed them in.
Tractor runs great. Now I have to pull it all apart again to fix the fifteen other leaks that showed up after about a half-hour of mowing! AND the twenty or so worn out bushings in the chassis. AND the wiring harness. AND.....
Hey! The tractor went for $15K new, and I got it for (net, now) $353.
LLoyd