6211 hydraulics not lifting to full height

Hi, i have a 6211 and it has never been any trouble it is only used for doing a bit of grass cutting. just recently the link arms wont lift to full height and will drop while in us when the tractor has been working for a while. The oil in the back end is fully topped up so that is no problem, and i have have checked the linkage on the lever and this is getting to full height but he arms will not come all the way up. Is there any way i can test the pump with gauges to see if its putting out full pressure and if so where can i do that?
Or does anyone have any other ideas what the problem may be?
Cheers
Chris

Probally the change over pendulum slipping off the roller pin.

Hi, how do investigate this?
Cheers

Take top cover off & look at linkage