next stage...
 

the engine started perfectly first time with two breakers yard batteries and some fresh fuel. :)

when I run the engine, the pressure for the air brakes builds to about 4 bar nice and quickly, and then starts to drop off. also the air is not reaching the tanks further back under the vehicle.

does anyone have a picture of the layout of the braking system please?

See:- http://www.truckandbusforum.com/gall...hp?i=32634&c=5

Is 58 psi enough to run an air brake system?

A 'protection valve' operates by limiting leakage (by 'protecting' the other circuits (reservoirs) to preset limits.

It sounds as if (maybe) the pipework supplying the air-doors is leaking and the normal supply to the service reservoirs isn't happening as the protection valve is preventing 'leakage'.

Normally, the air circuits would be arranged such that ancillaries (such as air doors) wouldn't be supplied until the service reservoirs were charged to at least the protection pressure.

In a working system, there should be enough air feeding the reservoirs to provide at least secondary performance before any air is 'leaked' to the ancilliaries.

Look for an airleak in other than the service-brake reservoirs (possibly an unprotected add-on or modification of the piping).

Of course you might have failure of both air reservoirs (such as missing connections or holed reservoirs) but it seems unlikely.

You might have to try disconnecting pipes to see which ones don't have air coming out and which one is leaking away the air after the protection valve has opened (at 4bar).

Don't forget my earlier advice to seek assistance from the local bus operator or a (maybe retired) mechanic.

It's almost impossible to diagnose from afar, but an experienced mechanic should be able to home-in on the fault quickly when faced with the hardware.

Is the footbrake valve jammed in the on position?
Air should only go to the servo when the pedal is pushed, but if it was jammed down the air pressure would never build up . . .

Gosh.

Thanks ever so much not only for the brake circuit diagram, but also for all the advice. Absolutely invaluable.

I'm going over again on Wednesday armed with this new info and a good engineer. I have followed your advice and found a local (not yet retired!) man who knows his Bedfords and does MOTs, and recovery. So the bare minimum I need is to be able to drive it 100 yards and out through a gate for his recovery vehicle. Preferable would be to get it to a good enough state to drive it to a booked MOT at his garage.

I'll keep you posted as to progress.
You are a most helpful man.
Thank you!
Mark

Encouraging news!

Keep us informed of the findings.

:)
of course G-CPTN!

Your 1985 PJK should have a triple-protection valve (as shown on the HMVF illustration) rather than the simple non-return valves on the main reservoirs.
Looking at the http://www.truckandbusforum.com/gall...3/imgp9440.jpg illustration will help you (and your mechanic) get an idea of the layout.
Ignore the extra components for the trailer brakes and the winch!

Your air doors should be fed from the third port on the triple-protection valve (which should be on a chassis sidemember somewhere between the condensing tank and the two main reservoirs).

It might be difficult to trace things unless you can get the bus over a pit or on a hoist, though you might have floor panels that can be lifted.

Who did you use?
 

Hi hope you got her all sorted. I have just bought a 1976 pjk dominant bus and not far from you at Bruton. Who did you use to do the work as mine needs a good service and mot after being stood for last 18 months.
Also any help from members as to who to identify engine, gearbox makes would be most helpful.
Thanks