08' Volvo VHD bad air valve?

View attachment 170709 I have an 08' Volvo VHD ten wheeler and it has three air valves right behind the cab. One air valve is mounted above the other two, the one that is mounted above the two is constantly leaking air. If I step on the brake pedal it stops, but if you hold your foot on the pedal it slowly starts to leak again.

I have no idea what the valves do and will probably end up bringing it to a local mechanic to get it repaired. My question is, is the valve just failing or...

08' Volvo VHD bad air valve?

08' Volvo VHD bad air valve?