I am constantly amazed at how much you seem to know about these old girls! Do you have one there or is it all in your head?

G-CPTN worked for Bedford for a while so he knows quite a lot, and what he doesn't know (not much!) he researches.

17 years as an Experimental Vehicle Design and Test Engineer.

For the benefit of others working on TKs, this is my understanding of the primary air supply system based on the discussion in this thread.

There are two pressure switches for low air pressure (one of which was not connected) that are in the two lines between the footbrake valve and the servo. Surely these lines see low pressure most of the time (except when the driver has his foot on the brake) so what are the switches for? I wonder if they are in series with the brake light circuit?

From where you describe the position of these switches I would say they are stop light switches.

I wish I wasn't so dim sometimes! That explains it perfectly; the low air pressure switch for the buzzer must be somewhere else, probably in the cab near the emergency brake release valve.

Sorry I can't help you with that one - I last worked on a T.K about 30 years ago and I just can't remember.

There will be at least two - one for each reservoir.

Sorry - I cannot identify the locations (electrician's job), at a guess look inside the chassis side-rails.

I'm fairly certain that they aren't an integral part of the gauges - but I'm not 100% sure.

I think it was only on later vehicles that the switches were incorporated in the gauges G-C. I remember the first time I came across them I was pulling my hair out trying to find them.

There are certainly no more switches in the pipework or on the chassis. They could well be in the gauges though, this is one of the last TKs (1981) and has the late dashboard.

It's a shock to me that I have forgotten so much about the mechanics of how vehicles work after spending the best part of my life repairing them. However thinking about your problem of the low air switches - have a look at the low air gauges and check the amount of wiring going to them. A simple gauge that just registers air pressure will only have a small diameter air pipe running to it and the wiring for the bulb that illuminate's it. A more complicated one that includes the low air warning system will have a lot more wires running to it.
However I might be giving you the wrong information here, but have you looked at the pipes that run from the air tanks to the foot valve ? The low air switches might be connected to those either directly or from a tee connector and small diameter pipe just above the foot valve ?

The pressure gauges for the two main tanks are part of a cluster that has temperature gauge, warning lights etc so a whole bunch of wire on the back. I'll trace the wires back from the buzzer some time but I'm pretty sure the suggestion that it is part of the gauge will be right; There aren't any other switches on the pipework.

Turns out there are 34 air pipes in a TK, or in my case none

You are, indeed, a brave man - I trust you are aware of what connects what to what.

I see that your vehicle retains the metal piping.

Was your vehicle built in Oz or the UK I am somewhat surprised that the pipework isn't plastic/nylon.

I labelled all the pipes (you can see the little tags in the photo) and took lots of pictures, then added notes to the pictures. All metal pipes buts as you say, a chat with the man down at our local truck parts store tells me I can replace it all with brass fittings and nylon pipe, it just pushes together! It was an NZ truck I think.