Having assembled a fair number of these at one time..
Depending whether made in Birmingham or South Yorkshire.. Subtle differences but basically pretty straight forward.
I seem to recall the barrel shroud being a little fiddly to fasten but besides that make sure all the Allen screws are back in the air cylinder. They hold the ends in place by being countersunk into the cylinder ends so the force is across the head not the thread.
I think the bit holding the gauge is aluminium with 2 o-rings, effectively floats, kept from sliding by the gauge..
I can't remember if that gauge holding bit will fit down the tube from the filler end but if it will there are less places from that direction which could knick the o-rings.

Obviously, you work on it at your own risk, hope this is of help.

Jonathan