Colin
I have had the adjustable ones , in fact I still have one and the problem is always the same, the metered draw in anything but accurate. In some instances the 5 quart bottle would take for ever and a day to be consumed, whereas on the very next job it would be gone in an instant .
And before anyone says it was faulty, it wasn't, it was simply junk.
I have on the other hand used many traditional Hydroforce's and rarly had a problem.