I have a similar problem - I suggest trying:
Turn the main house stopcock fully on
Wash the internal fittings in dishwasher when problem occurs
Always keep a "toilet blue block in place"
Add a dash of washing up liquid in cylinder every so often
Renew any plastic/rubber seals
The problem could be dirt (or bacteria if using rainwater) on the seal ring located low inside cylinder
best wishes Nyge C
March 2011