I recently refitted my winter tyres but when I came to check the pressures I found that I could not remove the alloy valve dust caps that I fitted last winter. With hindsight I should have realised that there would be a chemical reaction between the threaded brass end of the valve and the alloy of the cap also aided by salt from the road. I had to have the valves replaced at a tyre fitter at a cost of £20. The fitter tells me that after every Christmas they get this problem as children buy these cheap multicoloured valve caps as a novelty present for their fathers. Perhaps it would be timely to warn your readers of this problem.