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Bikes: 748S, HM1100S, V4SP, Was: DD-A #111
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Join Date: Feb 2002
Location: Back in Essex
Mood: Passion Killer
It's depressurising - the pump creates a vacuum in the tank and it has a valve in there somewhere to stop it from imploding due to differences in pressure.
You can sometimes stop it by simply opening up the filler cap and closing it again.