JCB 520-50 Brakes...Can they be adjusted?

My Dad has a 520-50 that is always a workout to brake. You get a tiny bit of brake response possibly w/ the pedal about half way down n maybe a bit more after that but basically you've gotta stand on it to get good stopping power. There doesn't seem to be much feathering it...you've either got the brakes locked or none at all. Can these be set up somehow? Is there a valve that could be sticky? I don't really know anything about the braking mechanism on these. Just beginning to think about tearing into it and hoping someone has already conquered the learning curve on this one.
Thanks.