Aftermarket master cylinder cover leaks?

[06VMAXIMUS]

Any of you guys have leaks on your aftermarket master cylinder covers? A friend has a set like mine and he developed a leak while the bike was parked on the side stand with the handlebars turned to the left. He noticed that the OEM covers have a vent to atmosphere and the aftermarket ones don't. The vent hole should only allow the gasket/diaphram to freely move as needed, and is never actually open to the fluid. I told him he probably had it over full but I guess it makes sense that if the fluid expands and contracts, it needs to be able to move. Could it build pressure in the reservoir and cause it to bypass the gasket? Any one run across this on their bikes?

 

[Vmaxnewfie]

I had the same problem a few months ago. I took the covers back off and cleaned the sealing surface with 400 grit sandpaper. Wash the rubber gaskets with soapy water and let dry. The clutch still leaked for a day or so after, I think it was a little overfilled because it hasn't leaked a drop all summer. Hope this helps.

 

[Fire-medic]

Just use a cut-off wheel to cut a groove like the stocker in the aftermarket cover.

Also, fluid doesn't 'expand and contract' to the extent that air does. For an increase in one bar pressure, water compresses < 5/10,000 of its volume. What does expand and contract are the gases in the brake fluid as it heats to boiling, which forms bubbles and the 'spongy-lever' (or pedal) on a motorcycle. This is much-worse on a master cylinder of old fluid, which is loaded with water, as unfortunately, brake fluid readily absorbs water, it's hygroscopic (silicone and DOT 5.0 excepted). Any firefighter should be able to inform you that water turning to steam expands to 1700-times its liquid volume, which is why it's so-useful in firefighting. It's the water in the brake fluid which is percolating-out as gas when the brake fluid becomes hot-enough to 'boil' that causes the 'expansion' of the fluid.

Guys used-to use a terry-cloth wristband on their master cyl reservoir to sop-up excess fluid which worked its way past the gasket while racing, before the brakes got better.

 

[06VMAXIMUS]

Good point, it is possible that the fluid has become old and moisture laden. I will suggest he change fluid and clean up the sealing surfaces. Thanks for the replies gents.

 

[johnpclynch]

Also, when you pull the lever and pump fluid down to the slave air will try get in to take it's place - it's better if the air circulates above the diaphram rather than below