A section through the brake servo:- http://s24.photobucket.com/user/Grou...P8509.jpg.html

I don't think there is a quick release valve on this truck, I think the parking brake exhaust is through the handbrake valve.

Here is the picture of the dual COV arrangement but I've labelled it this time.

The spring brake is on port B

Port A is a feed from the emergency brake release in the cab, so under normal operation COV2 always takes its air from port C

Port D is from the handbrake valve, port E is directly from one of the air reservoirs (one of the two footbrake reservoirs)

I don't see how the handbrake can be applied, since there seems to be always pressure in COV1. I've obviously missed something

Looking at your photograph, COV 1 takes pressure from port D and port E (alternatively) and then COV 2 takes pressure from port C and port A (alternatively) and delivers its output to port B.

The bit I don't get is this:

Driving along COV1 has pressure on both its inputs. Port D has pressure via the handbrake valve and port E has pressure from the foot brake circuit.

When the driver applies the handbrake port D is vented causing COV1 to take its input from port E and the brake stays OFF, still pressurised. I know this is not what really happens but I don't see it works. Port B has to be vented for the spring brake to apply.

Aha! there was a mistake in my notes, port E is fed from one of the lines to the brake servo so it only sees pressure when the foot brake is operated. Still not sure what this line does but I do understand how the rest works.

In theory, with the hand brake applied, pushing and holding the footbrake should release the handbrake; I'm not sure why you would want this but I'll have to give it a try one day when it's all back in one piece...

It is usual to have 'anti-compounding' so that brakes are not applied by two sources which could overload the mechanicals.

Oh I get it (I think) without this line it would be possible to apply the rear brakes with air pressure and the spring brake both at once, possibly damaging the parts.

As with most vehicles (although some folk don't realise) the handbrake serves as an emergency brake in the event of total failure of the service brake (usually split into service and secondary systems).

In the days before dual circuit systems, the handbrake (usually mechanical) was the 'last resort'. Although parking brakes are now typically spring operated, the control valve has to be 'progressive' such that the mechanicals don't break if applied when the vehicle is in motion.

I notice that the compressor pipe to the condenser tank takes a very long route. It goes right round the engine and back to a point on the chassis only about 2' from where its started although the pipe is probably 10' long. Do you think that is intentional for cooling?

Yes, the air is very hot when it leaves the compressor, and would rot the rubber hose needed to absorb the engine movement, also it helps to cool the air so that any moisture condenses rather than staying in suspension.