2nd hand car, with faulty sensor valves

Hi we currently purchased a 2nd hand car from a dealer and before we did actually purchase the car we pointed out things that needed doing which one was the tyres which were partly flat. they said no problem we'll fix that. 7 weeks later we are still having to put air in the tyres and have found out that 2 hav faulty valves. The dealer is no saying that this has happened after we purchased the car and the aa warranty they gave us with it doesn't cover it. What can we do as i know these were faulty before we purchased it ?