Extended Airport network

A=TimeCapsule connected to Internet

B= AirportExtreme, connected to A, secondary station

C= AirportExpress, new secondary station

According Apple http://support.apple.com/kb/HT4259?viewlocale=en_US

any secondary station must connect to the primary base station, hence to A

Problem:

when for C the signal of B is stronger than from A, then C connects with B. So connection is A -> B -> C.

When this happens C becomes useless, even when its LED turns green. Any device, even when in closest neighbourhood of C, does not connect with C, it tries with B or even A if possible.

C could connect with A, but B overrides with better signal strength and establishes the connection.

How can I force a network where C connects with A, not B?