[G-CPTN]

Originally Posted by mylesdw

Can you explain a bit about how the air brake foot valve works? It has two air supplies from two independent tanks and two pipes to the servo on the master cylinder. I'll grab a picture if you need it.

A dual-air footbrake system provides a safety feature (the system will work if either of the circuits fail). Likewise, the hydraulic master cylinder is split so that failure of either circuit will provide at least 'residual braking' for the vehicle.

A diagram:- http://lh6.ggpht.com/_Ii1ukGkfijY/Sq...jpg?imgmax=800

Compressed air from the compressor is stored in a wet tank in a semi-dried condition.
It then flows to the multi-circuit protection valve, which divides the feed to serve the two service reservoirs.
Simultaneously, pressurized air from the reservoirs combines through internal passages in the multicircuit protection valve to operate the remote spring brake actuator through the hand control valve.
Two service lines are connected to a tandem power cylinder controlled by a dual foot valve.
This arrangement maintains air supply to the other circuit in case of a fault develops in one service line.
The power piston push rod pushes the tandem master cylinder hydraulic piston forward so that the air pressure is converted to hydraulic pressure.
The hydraulic fluid supply is divided into two circuits to serve the front and rear brake expander cylinders.
In dual air and hydraulic line systems, both systems operate independently thereby safeguarding against failure of one or the other circuit.
The hand control valve is used only to park the vehicle. When the hand control valve lever is moved from ‘off to ‘park’ position, air from the remote spring actuator chamber is exhausted. This allows the power spring within the actuator to expand and provide maximum pull to the rod linkage of mechanical parking brake.

From:- http://what-when-how.com/automobile/...em-automobile/

Does that answer your question - or would you like a more detailed explanation?