Before you replace an air bag check the filter in the compressor assembly. This air cartridge can corrode inside as their are metal separator plates. These can be bought on line for about $45.00. If the filters is faulty can cause all kind of problems. The compressor is located in left front wheel well, towards bumper. Can be accessed by removing tire and plastic fender well, usually held by three bolts to frame.
It can be costly trying to fix a problem that your not quite sure what it is. When my Mark 8 failed were were in the middle of Christmas shopping. Little did I know it was going to take more than an air bag to fix the problem.
I got a conversion kit from Strutmasters. com and fixed it for good!... I saved a bunch of money too. :)