The International CBX Owners Association

Gave the front master cylinder a refurb with a clean up, seal kit and a new 'o ring' ( genuine and £8!!) , put it all together and it leaks from the join between the cylinder and reservoir, any one else had this and if so, how did you cure it?

Did you put a new reservoir in? If not there is your problem. Unfortunately you will probably destroy your reservoir but you need to pry it off and replace the oring with a new reservoir. $60 aftermarket

I did wonder about that, I'll order one up and see if it fixes it

The cup o-ring ALWAYS must be replaced in both the front and rear master cylinder or they will leak.

Dave

Seems that the OP put a new O-ring in there.
Did you put some brake fluid around the O-ring and make 100% sure that you didn't damage the plastic where the O-ring seals. I really had to really work at it and pry off the reservoir with 2 screw drivers and they were getting close to where the O-ring seals.
I tried to make sure I didn't score the plastic with the screwdrivers.
I just went through the ordeal of a new MC kit, but luckily I didn't have to replace the reservoir. Good luck.

You can get brake system-compatible grease that makes caliper and master cylinder assembly so much tidier.

Dave

Do not be tempted to use any old oring of unknown composition in this location. Garden-variety Buna-N (common oring material) is NOT compatible with DOT3-4 brake fluid, for long. Get the part made for the purpose, the OEM part from Honda is always a safe choice.

Just to emphasize: as Nils mentioned, common o-rings, which are Buna-N ARE NOT compatible with brake fluid DOT 3 or 4 or 5.2. They will swell and lock up in a short time. And, eventually, disintegrate. Same applies to even a static seal exposed to brake fluid. Swelling might not hurt a static sel. BUT,it will be negatively affected.