I own a 58 plate S Max and recently had the pleasure of fitting four new tyres. My S Max has tyre pressure monitors and I have found out the expensive way that when the tyres are changed that Ford highly recommend using a service kit to service the valves. The kit comprises a rubber seal and a locking nut.
My friendly tyre fitter ignored this advice and when I returned and complained about losing circa 10psi a day from all four (which had previously not lost anything before) he told me it was due to corrosion and the seals in the valves. They tyre monkey then managed to actually shear two of the alloy valves in half! The valves are £38 plus vat each from Ford and he is going to be getting the bill on Saturday from the dealer.
So, just a heads up to those with these valves fitted, its worth keeping them clean and free from corrosion, I would recommend a light coating of waxoyl or similar. Its also worth getting the service kits from Ford before you have your tyres changed.