"Why does the rear end lock above 20*?"
Speedpup is right, stability. A 4 legged chair is more stable than a 3 legged one.
"When you boom down and release the brake the machine jerks violently as the rear end unlocks."
The reason is that you have your foot on the brake. If you read the manual, which you agreed to do when you got your operator's license, you would see that the service brake actives the stability system on Lull C-series and newer (and Sky Trak legacy and newer). The exact functionality is somewhat different with the different models. It was found to decrease the chance of a tip-over because when a machine starts to tip most operators will stomp on the brake (unless they are too stupid to try jumping out).
On the E-series machines:
<40° - rear axle is in free pivo mode
Note: SN 0160003881 & before: depressing the service brake below 40° will engage the slow pivot mode. See below.
>40°, in gear, park brake off, service brake off - rear axle is in slow pivot mode.
This mode was created to satisfy operators moving hydro-scaffolding, which requires the boom to be >40°. Earlier models would be locked at this angle. Note that lifting a wheel while driving in this mode does NOT necessary mean that you are tipping over. If you put a chair on an uneven floor you will always lift the lightest leg. Lift two wheels =
.
>40°, with park brake on and/or service brake depressed and/or shifter in neutral - rear axle is locked to the frame.
The trick to minimize or eliminate the jerking is to go from locked to slow pivot mode before going to free pivot mode.
On the C-series machines the rear axle locks whenever the service brake is depressed. And no, they don't "build pressure" but what normally happens is...
- operator drives up to the drop point while partially extending/hoisting the load
- at some point the service brake or park brake is engaged. If one of the rear wheels has lifted the stability cylinder will lock it in the raised position.
- the load is dropped off and the boom is retracted.
- at some point the stability cylinder is unlocked and the weight shifts to the rear axle
- if the weight shifts too far back before the cylinder unlocks the rear of the machine will drop suddenly.
To my knowledge this isn't much different than any of the other machines with a stability system. In fact many of them would probably be like the Lull system except they put a patent on the functionality. Sky Trak kind of worked around the patent, but when they merged both brands moved towards a common functionality.
ISZ