D31P-18 and maybe other models have an inching valve on the top very back of the rear end housing in center. Its under the battery tray and has two small hydraulic spools that disengage each respective drive. They are pushed by cam follower type rollers, and with time the cumulative slop in control pins, linkages and bushings makes it so that the cam follower wont push the spool far enough. Or sometimes it will and sometimes it wont depending on the slop stackup. If you tighten the brake bands youll just drive through them so dont. You need to replace these worn metric pins with new stuff or atleast weld up and regring the grooves worn into them. I drilled most of it oversized to the first SAE dimension it came to which was only like .030 to .040" of material removal then install a bolt or american pin. Use soft stuff so it takes all the wear.
Dont be afraid to pull the inching valve right out, its all contained and you wont lose any fine parts but beware there are 4 small metal hydraulic feed tubes that have O rings at each end. Change em out, a universal O ring kit is fine.
Today i lost reverse only on my machine. I have forward in all 3 speeds and i have all 3 reverse detents on the stick. The rods are all there and moving the parts they should so its definitely internal. It used to take a while for the machine to engage reverse when cold.. Never forward, that was always fine and still is normal. But i have no engagement in reverse at all at any throttle. Im guessing an O ring in the control guts failed. Still pushes great going forward.
Any ideas?