A904 starts in 2, no 3rd , solid reverse

It is possible to put the 1-2 shift valve in the wrong way. The PTKD housing & end plate will still fit, but due to the contour of the 1-2 shift valve the channel that feeds oil to the front servo will be uncovered when the car is at rest, giving you a second gear start. I'm with Tracy & CudaFactHackJob on this, plus the fact you said your air pressure test checked out fine, confirms the fault should be found in the valve body.