On mine anyway, that 'connection' appeared to be a little loose on purpose to allow air in and out, so sealing it better meant I needed to add a vent to atmosphere. At the back now, there's a rubber vent fitting that looks suspiciously like a small tire valve with the core removed. There's a short coiled section of tubing on it for the duty, acting as a fluid separator and drain-back. Without that vent, any splash from returning fluid conspires with the air escaping from the reservoir to leave a film wherever the air can escape as you lower the lift.
Small tire valve comes from a parts store if there isn't one in your collection of things. I drilled hole as high as possible in the plastic reservoir housing. I made a wood support to let me lever-pull the valve through the plastic with a tire valve tool. The rubber valve seals in the hole nicely.
There are fancier metal-stem tire valves with seals and nuts you could use instead, but I had the rubber valve handy along with the tool to install it. Choose the metal-stem carefully, and you can fish it out through the new hole via the filler opening using the rubber tubing. Good if you don't really need to reseal the top of the reservoir to the pump.
The biggest 'work' was removing the reservoir without damaging the little molded-in lip that fits the groove in the pump housing.
*** 'Seepage' is a term adopted by owners of early and maybe some not-so-early British cars and motorcycles. Seepage is something you monitor and often feed to maintain. Contrast with 'leakage', which of course is something you'd need to actually fix. The difference is subtle but nonetheless important.
