probst JUMBOMOBIL JM-VARIO-150-B Operating Instructions Manual page 114

Operation
The distance „A" of the tappet at the upper side of suction plate (mounting side
operating valve unit) is adjusted to 68 mm (ex works) und secured with locknut.
Never change these distance, otherwise the load (kerb stone) could fall
down. Danger of accidence!
When engine breakdown the load/kerb stones does not fall down (check valve).
Residual vacuum lowers the operating valve unit/lifting hose unit with the sucked in load/kerb stone slowly.
Do not use the device to jerk [sic!] seized loads/kerbstones!
No work stoppage (pause) with sucked load/kerb stone, danger of the overheating of the vacuum blower!!!
Adjust engine speed in such way that a minimum vacuum of -0.42 bar is present (see  manometer at the operating
valve unit).
Mounting side operating valve unit
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