I've drawn the servo-hydraulic drive system.
Mechanical logic though.
Drawn as best I can.
Sorry about any offence to familiar conventions.

http://www.weldsmith.co.uk/tech/fatg...-hyd_drive.pdf

There's two "bobbin" valves.

The big one dumps the main power system oil to the tank.
It opens when the cylinder pressure exceeds the reference pressure.
That cylinder oil comes through a check-valve, so once pressurised and
vlave open, it stays open.
Until...
The other "bobbin" valve opens when the cylinder pressure is so low it
cannot overcome a spring - that opening dumping the ex cylinder
pressure holding open the main bobbin valve against the reference
pressure - that reference pressure reopening the main valve.

All this mechanism could be on one "pallet" - reference system and
power system adjacent, with one pipe to the test-rig atking
back-and-forth flows.