question defect in hydraulic valve on my URSUS 1224 (polish ZETOR licensed tractor)

Hallo friends,

I'm writing from Germany and own an URSUS 1224 (85kW / 115HP, ZTS-Martin 6-cyl. engine 6842cm, built 1990).
The tractor is similar to ZTS Zetor.

The first double action hydraulic control valve for accessories (nearby the control lever for the threepoint linkage) has a little defect:

When ok, the function is:

Pull Lever back: Oil to channel A, B opens back to the tank.
Do nothing: 0
Pull lever forward once: Both channels (A and B ) open to the tank, the controlled cylinder(s) can move free - floating position.
Pull lever forward twice: Oil to channel B, A opens to the tank.

The lever can stay detented in position 0 and in floating position (forward once).

Now the problem:
The lever suddenly does no more stay in position forward once (floating position), only detents in position 0.
When put into floating position, the lever comes back to position 0.

Does anyone know why the detent in floating position has this malgfunction? How to repair? Expensive?

Many thanks and please excuse me not being able to perform best english language here!

Greetings from Germany
Holger

Almost all of the Crystal range lose the detent after about 3000 hrs. Most users don't bother to fix it because everything else works OK.
The hydraulic top cover has to be removed to get at the external services valve, referred to as the outer circuit in the instruction book . There is no mention in the book of how the detent works.