I'm not exactly sure where the leak is you're referring to.
If it's the fitting itself, then check there is no damage to the flare. Then check the hose fitting for nicks or damage. Obviously, use two wrenches to tighten the fitting.
If it's leaking around the high pressure outlet and the canister, then the leak is because there is only now a stupid paper gasket between the canister and pump itself. There used to be a rubber and steel washer, but the rebuild kits only supply a paper gasket, which is basically useless. I have rebuilt 2 pumps now and in both cases, I had this gasket leak. What has to be done is the canister has to be removed. Make sure the base of the canister is perfectly flat and clean with brake cleaner. I cut my own new gasket from oil proof gasket material a bit larger than the original. Apply some "The Right Stuff" on both sides and fit over the outlet tube. Lube the big "O" ring with PS fluid and reattach the canister. I also ere on the side of prevention by adding a bead of The Right Stuff around the shell lip. Probably not necessary, but that's me. Tighten the large flat nut, let the right stuff cure and it should be good to go.
Arrow shows where the "paper" gasket goes.
EDIT: I must be blind, didn't see the pic posted.