It depends on whether you have hydraulic or air brakes in the wheels and whether you have a spring-brake parking brake.
I would assume (!) that your emergency brake release supplies air to a spring-brake chamber somewhere on the vehicle.
Some vehicles had a spring-brake chamber that operated pull-rods that applied the rear wheel-brakes by mechanical levers whilst some vehicles had a transmission brake (drum or disk) on the rear of the gearbox or front of the rear axle.
This might help:-
http://www.truckandbusforum.com/showthread.php?p=13927
http://www.greenmachinesurplus.com/b...1087-239-p.asp - or cheaper secondhand on eBay (carriage could be expensive, though).