what about tilt and auxillary functions like sideshift?
do they work?

gotta check and see if your getting pressure up to the control valve, don't want to go taking that apart for no reason ;o)

also check your suction hose and suction filter inside the tank.
also if it has an inline filter remove that and check it.

when you try and use hydraulics does the engine change like it's loading up? can you hear any sounds like whining like fluid cavitation coming from the pump?

all sorts of things to check out ;o)