To air check a transmission you leave the valve body off, use a diagram to identify the various clutch / band apply ports on the case then apply air using a rubber tipped blow gun. Some transmissions ( not make specific ) need a plate bolted to where the valve body would go as sometimes passages are not round.

When applying air , a slight hiss and a clunk as the element applies is normal. If you get a whoosh, that isn't normal if you are applying air to the correct port.

This isn't used to check pump capacity, the goal is to find something leaking excessively like a cut clutch piston seal / failed piston check valve.