It could be that one or more of the pump screws has come loose
I don't *think* the Cinq' pump screws screw into an open water channel (i.e. I think they are blind ends) but if one of them does, then your leak might be through the screw hole.
Anyways, take the pump off.. *keep* the original gasket. From memory it's one of those plastic/waxy "card" ones. Use some carb' cleaner on it to wipe off the gasket sealant etc. that you applied before (if any) and then when it's clean and dry, stick new sealant on both sides and re-fit it. It should be fine to re-use it.
I use Blue Hylomar for water pump gaskets... but if your pump screws are into an open water channel (i.e. the end of the screw hole is open) then you'll need something hard setting, like a silicone sealant, on the screw shank where it passes through the pump. Blue Hylomar only semi-sets. so it can get pushed around by coolant pressure.
I wouldn't use sealant only, if you have the gasket (even a re-used one). Some sealants only semi-set and the gasket helps keep it all together... and/or the mating faces would have to be dead straight and immaculately smooth and clean for the sealant to do the job by itself. The gasket in part evens out the variations between the mating surfaces.
Having said that, I have seen just sealant holding parts together.. so it may likely work without a gasket if you can't use the old one.... but they're only £1 or something... so why not?
Check whether the leak is coming from the horizontal steel water transfer pipe that comes from the radiatore. You might have dislodged some ancient rust or dirt that was keeping the pipe watertight. The metal pipe has a round O-ring on the end.. if you removed it when you fitted your pump, it may have snagged/been cut when you refitted it, so the leak may be from that joint, rather than the pump.
Ralf S.
