Hi, I bought a used troc tdi 2019 from a main dealer three weeks ago - 19 inch alloys. On the long motorway drive home, tyre pressure light came on. Checked and it was 12 psi and reinflated. Took the car to a tyre shop, no leak found. Resealed and new valve. Couple of days later, again tyre pressure light. Dropped to 31 - from 35. And again a few days later. Dealer paid for a new tyre but again they weren’t able to find a leak in the tyre fitters. One week later, again, tyre pressure warning light again and the pressure dropped to 31. I’ve had the wheel into a wheel specialist today and they can’t find a leak or issue with the alloy - they’ve had it in a water bath and put 60psi in. Main dealers want proof of a leak before they’re prepared to replace the alloy. Could this be something else or a mechanical issue causing a loss of pressure on that rear left? Where do I stand with this main dealer given I’ve only had the car three weeks and they know it’s been sold this way to me. It’s becoming very frustrating, it seems that the light tends to come on on long motor way drives, making me think the heat possibly is making it expand and leak and maybe why they can’t find it in the tyre/wheel shops. Thanks for any insight and help 
