you mentioned earlier about bubbles of oil (foamy) coming out of the dip stick (fill tube)

after doing some checking i noticed in the maintenance section of the service manual where they give the oil specs for the different systems. It does not specify an oil type for the yransmission, all it has is a hyster part number 336831.

now i'm going to make an assumption that hyster wants this specific oil in this transmission, if you put anything other than that in there it could be part of the problem.

The reason i question this is because some oils they recommend or insist on using have certain properties such as low sudzing agents or a specific type of synthetic agent to help the components last longer due to clearances etc.
I suspect this oil they recommend is a low sudzing type oil that will not bubble up during circulation back into the case.

It's just a thought i had because of an issue i had like this with a yale truck a few years ago....
We thought using an aftermarket brand of oil that was supposed to have the same properties as thier recommended oil would work but it did not. We wound up having to use thier oil they listed.

If this is the case with your situation i would try that first if you did not use hyster recommended oil by that part number.
If you did then just ignore what i just said and pull the tranny ;o)

good luck