Hello yoyocint
Sorry for the extremely long delay in replying; to be honest I'd more-or-less given up on the Monks machine until very recently - I've simply too much else to do and despite thoroughly servicing and testing the pump, and replacing all the tubing and thread with Monks own spare parts, I was still unsuccessful in getting reliable pull-through of the thread.
I took the Charles Austen pump completely apart (a gent at the manufacturer's very kindly sent me both the data sheet and service instructions), cleaned everything again, checked the diaphragms (which are fine) and replaced the neoprene valve discs as a precaution (although the old ones also looked fine, and actually work just as well as the new ones). I've bought a simple vacuum gauge and checked the level of suction; at zero flow I get a maximum vacuum level of about 21-22 in Hg (about -0.7 bar), at both the pump inlet and also confirmed at the nozzle with all the tubing and the waste jar in circuit - so there are no significant leaks anywhere. This level of vacuum is actually somewhat better than the pump's data sheet figure of about 17.5 in Hg. And with the pump on the workbench, operating in the closest I can get to free air conditions (i.e. with negligible inlet and outlet pressure drops) it fills a ~10 litre polythene bag in about 20 seconds, confirming an "open" flow rate of about 30 litres per minute, in agreement with the data sheet curve. So whilst these two tests don't fully check the intermediate pressure-flow behaviour of the pump, it does seem to be performing as it should. {And see also below, re. the flow and pressure with everything connected up.)
The one remaining but serious problem is that the buffer thread can be in a loose loop under the cleaning nozzle (having pulled it through by hand from the spool via the funnel and teflon feed tube) and although there's a full length of thread going through the nozzle and all the way into the waste jar, the thread still clings tightly to the walls of the arm tubing, and mostly refuses to "flow" into the waste jar. This problem gets noticeably worse once the thread gets wet with the alcohol/water mix (and incidentally isn't alleviated by adding a touch of PhotoFlo to the chemistry). The arm tubing is Tygon S3 E-3603, which was supplied by Monks directly and therefore should be the "correct" replacement.
The result is that although the thread is free to move before it enters the cleaning nozzle, and can even be hanging loose in a loop under the nozzle as it's paid out by the reel motor, it seldom gets drawn through to waste, and it absolutely never moves at all whilst the nozzle is in contact with the wet disc. Under these conditions the portion of thread under the nozzle therefore never gets renewed at all, unless I lift the arm and disturb the thread and/or nozzle (e.g. blocking and unblocking the nozzle with a fingertip to build up some additional vacuum and then release to get a surge of flow).
Apart from the arm tubing - which is, as I mentioned, of Monks own specific supply, the tubing from the waste to the pump is actually of larger bore and thicker walls than the original, so I don't believe there's any issues here. (I replaced the old stuff just in case, using a somewhat bigger diameter, simply in order to help slightly reduce the pressure drop in places where it can do no good.)
With the vacuum gauge I obtained from a supplier on eBay [see e.g.
https://www.ebay.co.uk/itm/274806170260?hash=item3ffbb96294:g:VFYAAOSwim9gqFJq ] connected via a T-piece at the pump inlet (also from eBay), the actual working level of vacuum at the pump, when everything's connected up as intended and the nozzle is off the disc, is about 12 in Hg (circa -0.4 bar). The degree of vacuum rises to about 15 in Hg (-0.5 bar) when the nozzle is drying a wet LP, and all the way to the pump's capability of ~21 in Hg (about -0.7 bar) if I seal the nozzle with a fingertip and wait a few seconds. So there are definitely no significant leaks anywhere. I did wonder about the soft arm tubing getting squashed a little where it competes for space alongside the teflon thread feed tube inside the arm, but if I replace the arm tube with a similar length in the open, so to speak, and connect it to the same nozzle, I get just the same pressure readings as with the arm tubing. With the arm and nozzle all connected up as normal, the resulting air flow empties my (not very well calibrated!) polythene bag of its roughly 10 litres of air in about 45 seconds, suggesting a flow of around 13 litres per minute - which again looks pretty much bang-on the Charles Austen F.65 pump curves (for the low-pressure variant with both heads in parallel). So again I'm inclined to give the pump the benefit of any residual doubt.
All that said, I chanced to locate a brand-new pump of very similar spec on ebay today, at a very good price (£50) and so I'm going to have a go with that. I'll let you know. Hope the above information is of some passing interes and maybe even helpful.