2007 Peterbilt 387 primary air buzzer stays on

Here's the million dollar question: does the primary air tank actually have air in it?

 

Put a pressure gauge to the different tanks and see the pressure. Looking for the primary tank. It may have red hoses going to the foot valve and or rear air tanks.
If there is no pressure to that tank look for a problem in its supply line.

 

Is there air pressure at the line going to the gauge? I am guessing not. Trace that line back and look for issues along the way. Has there been any work done that the air line could have been damaged?

 

you are supposed to have a low pressure switch near your brake foot valve. Use a multimeter to check continuity with and without air. If the result is the same, change the pressure switch .

I've seen some models with passthrough quickconnects, it is so much expensive that its better to install a pressure switch beside it.

Best of luck

 

Oh I meant under the hood, where all the air hoses are plugged on the foot valve