Stan and Jack have it right... the DD3s should set the emergency brakes at a preset pressure 45-60psi, and anyone that has DD3's should do routine pretrip brake tests that include pumping down the brakes and making certain that this safety function is operational.
My old MC8 had DD3 issues and I rebuilt one of the cans. Troubleshooting them can be confounding, but mine ended up being a tear in the aux diaphram of one the cans that kept the parking brakes from releasing. After digging around in it, I found them to be an amazing piece of engineering, IMHO.
The DD3 system relies on inversion valves that work in conjunction with the push-pull valve. There are then ball bearing-like "cams" in the cans to lock the pushrod in place to hold the brakes once air is removed from the system (there's two diaprhams and three chambers in the can... hence the name). Also, the DD3's do not
have the massive springs inside them that have to be caged to prevent injury/ death to the rebuilder the way spring brakes do. If there's three airlines going to the can, it's a DD3 system... and it's a great system for busses, IMHO. I've read that spring brakes cannot equal the force of emergency application that the often-maligned DD3 system can. My new bus, the 4108 has the DD3 system and it makes me feel a LOT safer than the old Johnson bar/ICC system in our 4106 (where you lose all air
if you lose a rear line!).
Some DD3 info from Bendix: http://www.bendix.com/downloads/service_data_sheet/024600.pdf
Just rambling on...