If the zone valve is a pig to get at it may be worth while trying to replace just the motor on the valve. Works a treat on a honywell two way zone valves. These motors are called synchron motors and you can get them on ebay for around £10 you will pay a bit more at Plumb center the beauty is that there are just two wires to connect . If you live in a hard water area it may be worth replacingthe whole unit.
Tony
September 2007