904 with Turbo Action shift kit, no 3rd

The front clutch is also active in reverse. If it were leaking you either wouldn't have reverse or it'd slip bad in reverse. If the front servo was leaking you probably wouldn't have 2nd gear because the front servo applies the front band which gives you second gear. I realize dropping the VB is a hassle but you really should verify the large check ball in the valve body channel is 11/32" as recommended by several guys already. If you mixed up the 3/8" line pressure ball with it things will go together but you won't have high gear. The other recommendation to check the 2-3 shift valve by Bronzebee is valid also. Either of those would most likely be the reason your not seeing any pressure at the kickdown release port because the only time you'll see pressure at the release port is when it releases, in other words when it shifts into 3rd gear. If it's staying in 2nd gear it's not releasing.