I've had it with this 904, about to give up!
Just the air check? I did that several times when I built it and it worked great. I thought we were talking about line pressure :P
No, a mainline pressure test is done with the trans all together and the car running.
It tests the pressure that is being put out by the pump and pressure regulator valve.
One of the ports on the side of the trans is where you put a pressure gauge, and you can move the TP linkage and should see the pressure rise and fall.
I think it's the middle plug on the passengers side that is mainline pressure.
I know the TF2 kit tells you to adjust the pressure regulator valve on the valvebody by adjusting the allen screw.
Clockwise lowers the pressure, counter clockwise raises it.
Could it be that it was adjusted the wrong way?
Also as mentioned above, are you sure your TP linkage went back on the valve correctly?
Can you move the TP rod forward and backwards?
Try tying it all the way back for a quick test drive and see what it does.
It should go into gear real solid and not want to shift out of first.