That's what I was thinking too. The excess play may be a sign of things being loose back there.
Also, the governor support itself is prone to grooves wearing in it where the governor sealing rings ride causing it to not seal good in newer 904's. I haven't had a older 904 apart with a rear pump apart so I'm not sure if their designed the same or not so this may or may not apply to yours. Sorry, I don't know about checking rear pump pressure. Have only worked on newer style 904's.