Most likely the filters in the captops have raised up or swollen up so that the head touches the filter and thus creates a syphon. If you replaced the head it may not be sitting the same as the old one or the captops need to be adjusted. This is a capping adjustment (flushing adjustment) in the service menu to set the heads on the captops. Make sure there is no ink that is left pooling in the captops when you move the heads off to look. If so then the drain line or the clear lines under the captops may be blocked and not draining the captops on cleanings. If left touching the heads it could cause contamination to the other colors or start a syphon again.
Ouch... that is a lot of ink to have run away.
Also make sure the drain lines in the drain bottle are not too much deeper than just past the top of the lid. Never let them touch the waste ink in the bottle, however after the first time I doubt that the drain bottle was still full.
With being converted to solvent, are you now just running lines from the pump to the drain bottle direct and not using the original thin drain lines that come with the water based systems? If you can see the drain lines you could put a small clamp on the drain lines to prevent this over night however if the filters are still touching the heads it could cause one color to then back up into the others and contaminate the line and if bad enough get back into the cart and contaminate the new cart or carts.
Only other things that I notice cause a syphon is some times one almost empty cart beside full ones. I like to use the 220 ml refill carts you can buy on ebay and just pour ink in as you need. They never pressure up like a sealed cart can and this way you can monitor what is happening and only waste small amounts of ink till you find out what is causing it.