Quote Originally Posted by ben2go View Post
Some don't have check valves. Usually, the master cylinder being above the caliper will do the trick. Also, the reservoir getting low can cause a bit of suction pulling the caliper in. The rubber seal doesn't let air into the system because brake fluid is hygroscopic and absorbs water from the air. As the fluid level drops, if the bellows in the seal doesn't drop as well, there will be a vacuum created in the master cylinder, which will pull the caliper piston back. This is why the master cylinder cap seal is made the way it is.
some dont have it eh... plenty of info on the web about other bikes wouldnt you say.... you had me running around for an hour looking where i could have dropped the seal