As you look at the front of your A4, the right hand tube has the lpr. The brass piston you see when you take the cap off may be sticking. This happens when the o-ring gets dry, and affects the ram pressure.
You need a 2.5mm screw or the angel tool, to remove the piston, but be carefull as there are cup washers behind that are in a specific order.
I would get a tech to check the A4 over, as it could be many things causing this problem, including your air system.