9540 hydraulic troubles

Well, 1st post for me, perhaps someone could shed some light on a problem I have, its a 9540 which I use with a post driver, however when its pointed uphill the hydraulics stop working ie I can't lift the rear linkage or use the post knocker. There is loads of oil in the gearbox... and when its on the flat or pointing downhill there is plenty of hydraulic power and everything works fine.
If any one knows of any reason behind this I'd be very gratefully.
Many thanks!

Oh yes and it seems to only happen when its warmed up, when I first start the tractor in the morning the hydraulics are fine uphill.

how is Your oil Level ? if you have steep land you can overfill the gearbox With at least 10 liters. i have heard of similar problems but then always With tractor pointing downvards.