My 57plate Viano started with bouncy suspension middle of last year, no fault lights but the rear air suspension controls weren't lit up and didn't seem to do anything. It got gradually worse until the kids wouldn't even get in it. Turned out the control unit under the jack in the boot had got wet and gone faulty, new one has been fitted and the ride is now silky smooth, 245 + VAT for the unit though.. plus some diag costs..
Good luck with it.