TK mystery part
 

Does anyone know what this metal bracket/shelf and thread stud are for?

This is looking forward at the rear of the cab behind the driver's seat.

It is where the hydraulic jack is/was stowed.

I knew you would come up with the answer Tango November! I wondered whether it was the washer bottle but that is mounted elsewhere. I might just cut that bracket out and use that area to remount the power steering reservoir.
Are the other brackets for a wheel brace or a jack handle or something (semi-circular brackets across the back of the cab)?

Many years ago I was following behind a TK when the jack fell out from it's bracket, it hit the road then bounced back up and knocked a hole in the diesel tank. The driver was unaware on this and carried driving along the road leaving a trail of fuel for some distance until he noticed me flashing my lights and sounding the horn.

Indeed they are.

Here is the power steering reservoir just below the alternator regulator on the old cab. Is this original? It appears to have been added.

Power steering wasn't fitted to KG models when built (nor was the 466 diesel engine!) so it seems that yours has been updated with parts from a KM.

Here is a KM reservoir:-
http://www.ebay.co.uk/itm/Bedford-KM...81235802481%26

Though local sourcing might look different.

I don't remember where it was fitted - sorry.

Not quite the same as that KM one but I guess it really doesn't matter. I've cut the jack bracket off now and the reservoir can go just below where it used to be. It will be very easy to get at there.

Do you know what this valve is called and what it does?

It is an assembly of two changeover valves.

A changeover valve contains a free-moving piston which moves according to the higher incoming pressure and delivers air to the central (outlet) port - which can become the inlet to a second COV (change-over valve).

They are arranged to operate so that a system takes air from whichever circuit is pressurised and can alternate footbrake and handbrake air - selecting the higher pressure without mixing the inlets or can use supply air from tanks or compressor to feed downstream circuits.

These COVs can incorporate pressure switches (see the flange on the upper COV) such as stoplamp illumination or low-pressure warning (yours don't).

Tracing the airflow of your illustration, air pressure would flow from either of the top pipes (straight or elbow) - depending on which is the higher pressure - to the lower COV.
The outlet of the lower COV (the middle horizontal straight pipe) would receive the higher pressure between the upper COV outlet and the bottom horizontal pipe with the elbow inlet.

Seemples . . . :D