Another master cylinder problem

[relisysxx]

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?

[tevan]

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

[relisysxx]

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

[daves79x]

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

Dave

[oldbikerdude]

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.

[daves79x]

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

Dave

[NobleHops]

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.

[Larry Zimmer]

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.