Ford Cargo HiAB - how to make it work?

Mad fool that I am, I just bought an '81 Cargo with fitted HiAB crane. The problem is, I can't make it do anything.... I've pulled the legs out & locked them, switched on the PTO as advised (clutch in, pull out switch) & I can hear a brief hiss as, I assume, the system gets air, but then nothing else happens at all....

Any clues? I can't find any plates relating to the HiAB, so I've no idea what model it is.