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Pull the cap off the reservoir and make sure the rubber diaphragm on the underside of the cap is not sticking out. This is the most likely cause as the diaphragm pushes the float down. The long term fix is to flush your brake fluid.

As the fluid's water content increases over time it will expand and contract more. On a cool or cold morning it contracts enough to suck the diaphragm down...at least that's my theory. Once the diaphragm is sticking down it stays down. <img src="/forums/images/graemlins/cheers.gif" alt="" />