Sealant and brake fliud

**tychver** · 11th July 2010, 17:09

What kinda sealant would resist exposure to brake fluid? I've cobbled together a good master cylinder for my CB250RS and it's working great except for one small problem: there's a very small leak around the base of the reservoir. It's not pressurized so there's no issue there. There was what I'd assumed was 25 years of brake fliud seepage sticking the reservoir down but now I think it might have been sealant...

**Max Preload** · 11th July 2010, 17:58

Personally, I'd be investigating the source of the leak a lot more carefully - if there's a crack in the casting it might fail in service.

**tychver** · 11th July 2010, 18:12

It's definitely just the seal between the revservoir and the master cylinder body. I've triple checked both

The fliud is coming back up around the reservoir as it sits in the body. You have to get the master cylinder on it's side and stand there for a couple of minutes before a bead of fluid forms so it's a really small leak. I don't know if it would get noticed in a WOF inspection but I still want to be on the safe side and sort it.

Diagram:
http://www.cmsnl.com/honda-cb250rs-a...slist/F04.html

**Pixie** · 11th July 2010, 18:35

Use a quality RTV silicone sealant like one of the Loctite products:

http://www2.blackwoods.com.au/infoBA...3621&P=2024340
http://www2.blackwoods.com.au/infoBA...3621&P=2024929
http://www2.blackwoods.com.au/infoBA...3621&P=2024928
http://www2.blackwoods.com.au/infoBA...3621&P=2024441
http://www2.blackwoods.com.au/infoBA...3621&P=2024930
http://au.iloctite.com/en/loctite-57...-518-gasketing

**tychver** · 11th July 2010, 18:36

Hmmm. I just checked on it again ~4 hours later and it seems to have stopped seeping when I wiped down and sat watching it for 5 minutes lol. It's way too slow to tell by the reservoir level. Could just be the o-ring needed to soak in fluid for a bit? It'd had a week to dry out. The manual says to soak new seals in fliud for 15mins but I didn't bother since I was reusing old ones.

**schrodingers cat** · 11th July 2010, 18:50

Originally Posted by tychver

Hmmm. I just checked on it again Could just be the o-ring needed to soak in fluid for a bit? It'd had a week to dry out. .

Unlikely. You'll have to get it all clean and dry (brakeclean) for the silicon to stick so why not just fit a new o-ring?

**tychver** · 11th July 2010, 21:39

It's still holding so far

I'll keep an eye on it over the next few days.

Originally Posted by schrodingers cat

Unlikely. You'll have to get it all clean and dry (brakeclean) for the silicon to stick so why not just fit a new o-ring?

I've already contacted Econohonda about getting one but my bike is my main transport so it's not really a great situation to be waiting on it.

**imdying** · 12th July 2010, 09:23

No sealant is appropriate for that, just replace the seal.

**tychver** · 14th July 2010, 13:04

Stopped dripping

Looks like what happened was I fitted the reservoir using the original 0-ring and loads got past. Then took it off and fitted it with another O-ring in better condition but a load of fluid was sitting in this pocket in the casting to the left of where the reservoir sits which of course continued to come out even once I'd got a proper seal.

Still ordering a new reservoir, cap, screws and o-ring to be on the safe side though.