This job has been a lot easier than I originally thought, the main cause of the problem was fibre washers in the pipe joints were either just worn or had become two compressed and got loose.
Apparently I am told that this fibre washer system is an old way of doing it, you would think that by now someone would have invented a better material to do the job, but it appears not.
So I got a big box of fibre washers and pipe fittings and doughty seals and off I went. It's all a learning curve, I have never worked on pneumatic systems before.
I am sure it would be easier if the fittings were all BSP instead of Metric, it is very hard to get metric fittings here.
I also managed to tidy some of the wiring that seems to have evolved on the truck whilst I was at it.
It's looking a lot better now.
This is the old valve, i removed it cleaned and painted it the refitted with new seals