I would bet that this truck has a Bendix AD-IS or DRM air dryer installed. If it is, it is
normal operation.
For the AD-IS see page 4 where it says
106 psi: click here (be advised these pressures can be variable)
For the DRM see pages 3 and 4:
click here (as I recall the DRM was developed specifically with kenworth in mind.)
Let me know if this fits with what you have.
Birken