Hi,

Thanks for the reply. Found the serial number along the right hand side of the engine compartment. The serial is E3000265

Issue seems to have cleared itself for now; removed the pressure piped from the orbital unit and started up, to check the flow then re-fit and the steering is working normally again. If anyone does have a diagram of the system it would still be appreciated as the issue may well come back

Well if it says yale on it and has eaton on it then there is no question....it is a yale :o)
Now as for the serial number location there a couple places to look for it. First place would be on the frame right around the right side step area, it would be stamped either on the top beside where the floorplate edge is or on the side of the truck frame in the same general area.
Another place to look would be on the front of the cowl on right side along the bottom edge between the mast and cowl area.
A third place might be along the right side engine compartment along the top about where the edge of the hood is.
It sould be either a 7 digit beginning with a letter... or a longer number at least 13 to 15 digits, starting with a letter like G and ending in a letter that could be any letter.
I also should tell you they did not stamp these sometimes very well so it may take some scraping, if it has been painted then that just makes it more fun to find :o)

Good spot, the owner said it was a Hyster. Still not able to see a serial number. I suppose we better put the post in the Yale section. Thanks for your help

Sounds like you have an old Eaton yale. Not a hyster. Serial should be stamped in r/s around floorboard/step area.

without a model number this may prove difficult to even guess at

first determine if this has a pump mounted on the engine for power steering or if it uses the hydraulic system

an easy way to determine this is to trace the lines from the steering orbital valve
there should be 2 lines to the cylinder, 1 line from the pressure source and 1 drain back line

some older units had a relief valve on the orbital valve, if this one has that then pull it and check for broken springs.
other units utilized a priority valve mounted between the control valve auxillary pressure outlet and orbital valve where it got it's pressure from, they are known to stick and cause steering failure.

and if its a really old design the steering cylinder is mounted in the frame along side the engine with a drag link and mechanical steer axle, those cylinder assys had a valve on those that controlled steering direction and if they failed the steering may go out.

without a model this is just a best guess at what to look for