So, having had some time away, looked at my Canon kits again and realised that the rules for Canon and Epson CIS kits don't necessarily apply to the HP kits. In fact I pretty much had to chuck common sense out the window on this lot.
Rule 1.
The HP K550, L7000 series and K5400, like the earlier officejets use what is essentially, built-in CIS system. The cartridges do not move and the print-heads are connected via a set of tube (much like a normal CIS).
Difference:
In Epsons and Canons the cartridges sit in a carriage unit which moves back and forward with the printer.
Rule 2.
The HP print heads have a small amount of ink in the printhead so if you get air in them it's virtually impossible to purge this doing a number of prints... The air will be retained in the printhead reservoir so the only option is to buy a new one if air gets in.
Difference:
In Epson, Canons, the printhead hold virtually no ink at all as the cartridges sit above the printhead itself. Thus if you get air in the printhead you can (assuming it's not burnt out) purge the printhead using any number of techniques discussed on nifty-stuff.com or using cleaning cartridges.
Rule 3.
The HP cartridges have a pump/piston activated system that pushes ink into the printhead. This is facilitated in the HP oems by the rubber bulb in the base of the cartridge and a piston in the printer. This piston forces ink into the printhead.
Difference:
In Epson, Canons the ink is drawn into the printhead through capillary and gravity action. There are plenty of instances where increases in pressure caused by placing reservoirs too high, etc... are known to flood the printhead killing the jetting process so there's definitely no "forcing" or "pumping" of ink into these printers.
Rule 4.
The HP cartridge consists of a bag of ink and absolutely no air hole.. The bag "deflates" as the ink is used up.
Difference:
In other systems the cartridge has a vent hole which allows air into the cartridge to balance out any air pressure differencials as well as replace the ink as it's used up.
There are plenty of other rules or bits to note but these in particular are important when it comes to the HP printers...
Now, having spent far too much time on the HP L7680 with my modified cartridge system I'm now pretty sure that my issues stemmed from Rule 3 (insufficient pressure) and Rule 2 (printhead design). By starting the design with a printhead which had already started to gain a little air and placing the reservoirs at the same level as the cartridges I was just creating a system that had insufficient pressure and once air was in the system, any adjustments to the ink level was a waste of time as the damage was already done.
If anything, hindsight seems to indicate I should have increased the reservoir height considerably and tried with a much increased positive pressure on the modified OEM cartridges. Hardly surprising though that the problems I was having with printheads made me balk and decide to drop the experiment for now.
Anyway, in the end it's somewhat moot as I've found a supplier of kits for the K550, L7000, etc... and earlier models that use the same system. Best of all the system doesn't even come close to using any of the pitifully small clone CIS systems on ebay, nor does it abandon the original cartridge design.
I've placed an order any will reveal my source sometime soon, or more likely they'll advertise themselves once their new site is fixed up
but either way it seems they do everything I wanted to achieve with a whole lot less mess _________________
Printers: (Canon) MP500/830, MX700, iP4000/4200/4300/4500/5200, iX4000(A3) (Epson) C84/86, D88, CX6600, R285/800/1900 (HP) K550, K850, K5400, L7680
