This post from an American forum:

Tisco used to have a set of Ball Valves that would replace the old
mushroom/automotive type check valves in the Massey Ferguson hydraulic
pump that ran in the bottom of the sump. An installation of these
valves would make a 3pt lift hold up for days, and only lower when you
started up the tractor and lowered the lift lever.

I've never heard of such things. Has anyone else?

JV