i know this thread is fairly old and you probably already figured it out but just for reference to anyone that deals with jlg units and the emergency lowering systems there are 2 steps you must take to get it to lower correctly
first there are release valves on the base of the cylinders that have to be opened
secondly pull the lowering valve

now once you get it lowered you can then close the valves and it will operate normally , if you try and lower it without openeing these valves it will now lower correctly or at all

these valves come in 2 types that i've seen, one is a black **** on the square base and the other is a red twistlock type **** located in the lowering solenoid on the cylinder base block.