Leaking air at back of 13 speed, what's this?

That is an air actuated solenoid... Not familiar with the workings of transmissions to know what it does exactly, but it probably has some bad o-rings in it that is causing it to leak when it shouldn't. I wouldnt imagine it would be to expensive to have fixed.

 

Insert valve.

 

Does it not just unscrew and a new one can be installed? Most likely the problem?
If it still leaks buy all the o-rings and gaskets for that cylinder and housing. It it not a very big job but it will dump all the oil.
You maybe able to get the piston out without disturbing the housing mounting gasket but do not bet on it.

 

I have actually had manual transmissions fail when a small plate a exact size of a cigerette pack cracks in half about that area near the shaft output in the rear. Your splitter goes away and most of your low range too.

Its been too many years gone by to be more specific. But the immediate problem was usually a 15 mph escorted slog back to Searcy on the old road. (367 which was the main trucking route before 1962 here in Arkansas...)

They kept telling me to check that #### plate in pretrip. It's a small but very important detail looking for Mr Crack working across that steel pending failure.

 

Nothing a few O-rings in a little bit of silicone grease will fix