Lozza,
I had a similar issue with the gear not wanting to lock in the down position. I ended up grinding a flat spot at both pivot points. Those being shown in your pic as the black and red lines. I found this by using a sharpie to make a line across the pivot and arm. If it's slipping then the line becomes separated, or not a line anymore.

For the 3rd cylinder on the mains...I am not using it. Couldn't find a way for it to be effective (work) without adding another valve/sequencer. If your main cylinders are not leaking then you will have plenty of air pressure to prevent a colapse.

Craig