Did you try putting petrol into it recently
Only joking, it may well be the float. If a hole comes in the float with the older type that use a brass type float, they can hole over time.
The result is that the float does not float (go figure).
I think you can get integral Float caps to replae the existing cap,
You may be able to fish out the old float for inspection and replacment.
Depends on the type.