Q: oil leak thru transmission breather vent

There is no thermostat. To diagnose this, you should install a temperature gauge to determine what the operating temperature is. The A140E tends to run hot already and if the transmission is failing, heat generation could increase due to slipping clutches. If the running temperature is above 200 degrees, that is too high. Be sure the cooler actually flows. If the situation is marginal, you could try adding an external cooler but that will not resolve the underlying problem and is more of a temporary measure. If the valve body is worn and causes hydraulic leaks that lower available pressure to keep a clutch locked, yes that could be a cause but there are many other, as likely, internal hydraulic leak points. I would start by determining the operating temperature. These transmissions are easy to rebuild (except for getting the driven gear off of the pinion which requires a 10 to 20 ton puller and a press to re-install). If it is worn out, just rebuild it. If you are interested in a transmission replacement, YourMechanic professionals in certain locales can assist with that. Please simply inquire based on your locale. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.