On this model of Q7 there are two lights at top of dash showing compressor activity, one green and one yellow, if these lights are on or flashing it shows the compressor is working to try and make up for lost air, if you let this happen for extended periods without solving the problem it will burn out the compressor, if the front is down evenly on both sides it is not a bag or level sensor, they are independent on each corner at front, the issue is with valve block or compressor, if the comp will not go full lift it is not producing enough air and the front shows it first because Q7`s have active ride level which works harder at front when braking, Compressors range in price from suppliers and on ebay from £189 at Maxspeedingrods to £800 at aerosus for a good reason, "they are all those products are worth" I am curious as to the unit fitted to your truck in last six months. Personally I rebuild all mine using parts from "Bagpipe Andy" inc new head and piston, air dryer and pressure relief valve, you are probably running at about 5 bar and a good rebuild should give you 11 bar. Take a look at the attached pics of before rebuild and available height after and tell me if you recognise the before as like yours