It's a long time since I built the pump and as far as I remember it was a bit of a waste of time with all the separate bits and pieces.
Major Arnot no doubt had his reason for designing it as such.
I make all my ram pumps as per K.N.Harris MODEL STATIONARY and MARINE STEAM ENGINES there are not so many parts in them and easy to maintain.
The only difference that I make is that I don't have a nut with gland packing, I use a Silcone "O" ring on the end of the ram and cut down the clearance on the groove width as no heat is involved to expand the ring, so if the ring is .070" dia in cross section I machine the grove to .080" wide giving .010 clearance but keep the O/All dia as per the ring machining instructions..
Here is some pics of a ram made for a high speed engines oil pump witch was geared down 125/1 from engine that did 10,000 rpm on the bench.
This one is for the water pump that had a hand operated pump to feed the boiler.
This one on a 10v with Scotch crank.
I hope that this is of interest to you.