Repair kit includes a windmill diaphragm, 2 O-Rings and 2 valves. Everything you need for routine maintenance on your windmill.
Typically this sort of maintenance is something that needs to be done every 3-10 years. Of course, this depends on how much use the windmill gets; with higher wind the faster these sorts of parts can wear out.
Below are the windmill installation instructions. Please let us know if you have any issues with your installation.