Bedford TL Power steering?

I have a 1984 Bedford TL horsebox. The Power steering suddenly gave up completely. Thinking it would be something to do with the fluid (and being rather dense since when you turn the steering wheel normally, you can hear air puffing) I soon realised that it was an air system. This is, as I'm sure everyone but me knows, an after market bodge up. It should have a hydraulic system. anyway, we are find it v difficult to find the fault, presumably a joint, or pipe (or which there appear to be many, all coming to the steering column in the cab and helpfully scribbled with Tippex) the Steering colum says Autosteer, which also no one knows about. Anybody at all got any thoughts? Somebody said drain the air system of condensation first, in case the brake system is saving itself by cutting off the steering system. Does this sound likely?
Anybody know what make of lorry it might have come off? I can find know reference to any air steering systems except some american military thingy. Any thoughts v much appreciated.