Float switches can get sticky with age. Easiest solution is to disconnect it (at the top of the bottle) and resort to checking the level visually about once a month.

The float switch is, however, easy to replace I think they cost about £12 here in the UK.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)