Here's everything I know about it:
Ariens Sport Zoom
Model 915031 Serial # 010072...
Looking at the service manual. Different type of setup then what I had in mind. Instead of a single pump running two hydro motors it is two separate motor/internal pump combination units. So disregard the one statement I made above and have quoted below highlighted in bold print. One unit could be bad and the other one fine.
I suppose you could try fresh oil and purging the air from the system. But if that were the issue you'd think both the drives would be affected.
Next regarding this:
Is there a separate bypass/bleeder valve on each transmission? Maybe it is not closing tight on the weak side?
The service manual shows a "dump valve" on each unit operated by a single lever so that answers that question as that is the valve I was referring to. Might want to check that when the lever topside is moved to and from the free wheeling position the valve levers on the hydros are both
moving with it.
I found an exploded view of the hydro. The plug you mentioned on the tubes also functions as a vent.
Still have not found a servicing manual for 01597800 (#346-2400L)
Looking at another manual (for a different model) it looks like to check the oil level the vent tubes and fittings are removed and you measure how far from the top of the transmission the oil level is setting. That measurement though depends on the model.
Wonder if maybe the one that is leaking is just plain a little low on oil causing the problem?
Here's a service manual for a hydro similar but not the same:
http://www.mymowerparts.com/pdf/Hyd...2600 Hydro Gear Service and Repair Manual.pdf
They have a number of other manuals as well.
"K&T Parts House Lawn Mower Parts and Chain Saw & Trimmer Parts"
Looks like to outright replace just one of the units is $700-800