What a hassle it was to top up my Isomac reservoir. Take all the cups off the warmer, remove the lid (usually too hot to handle) , pull out the pipes, fill the reservoir and then reverse the process. To solve the problem I drilled an 8mm hole in the lid directly above the reservoir and using a piece of silicone pipe and a 5 litre water bottle which was the convenient height set up an natural siphon.
Initially I had a bit of a problem getting the siphon going so I added a separate short section of pipe in the lid top. A short blow on this got the water flowing. This also acts as a breather for the water bottle.
All I now have to do is keep the water bottle topped up. (ensure the water bottle is not taller than the top of the reservoir or you could have a flood)
Hope this is use to anyone with the same problem.

I just got one of those rectangular 2.5 gallon water bottles and took the spout out of it, and replaced it with an extension (same silicone tubing) of Silvia's pickup tube. Extending the OPV's return line the same way, and tapped a hole in the top of the jug for it. Near a month of Silvia use (she was the 'spare' at the GF's house) before refilling.