Kenworth w900l air compresor?

Hello Iam Martin
I am from Europe and have owned a Kenworth W900L, year of manufacture 2003, Cat 6NZ engine for 5 months. I am putting the car together over the winter so that I can drive it in the summer and make some money. I want to ask the owner of the Kenworth something about the air.
The car inflates to 120 psi, deflates to dryer air and the pressure slowly starts to drop to 90 psi where it stops. After stepping on the brake it starts to inflate again to 120 psi and this repeats. It inflates to 120 psi and in about 30 seconds the pressure drops to 90 psi, no leaks anywhere. Is this normal? Or is there a problem with the Bendix D2?

Thanks for the apology for the translation, I prefer to write this through a translator.

 

Ok, thanks for the reply, I'm currently putting in new bags, so I'll try it out once I have them installed.
It's true that the filling valve on the bags works a bit strangely and quite slowly, it's possible that it might not be sealing.

Thank you