Sold 16 cornies to go down the Slimline Sanke route . . . A Couplers were £6 each! I have a filling Coupler and a Gassing Coupler, when you have JG fittings on the inlet and outlet it's easy to pop off a line and insert a blanking plug. I Fill with fined beer, gas as required, I tend to add all additives into the Conical FV after dumping the yeast so not an issue, being able to open them. They work and seal with a cask aspirator, so ideal for hand pulled beers ... the 1/2" dip tube is a big help on this as it offers virtually no restriction to beer flow, which means a partial vacuum isn't formed causing CO2 breakout in the line.
Remember Cornies are meant to dispense thick soda syrup at high pressure ... That's not necessarily what's happening in the hombrew environment