air pressure problem need help, 2004 volvo vnl 780
I have a 2004 Volvo VNL780 with a Cummings ISX motor, I am having issues holding air pressure.
I can build air pressure when driving at Highway speeds, while under load pulling with residence (ie. up hill)
I have replaced the Air Turbo Valve on the Air Dryer, as well as replaced the Cartridge.
Also replaced recently the Safety Manifold and air drain valves.
I have used a shop compressor to fill the tanks with the truck not running but assc on and found no additional air leaks.
When truck is running I rev the motor to 2K and can only build air pressure to 70 PSI ish, before fan cuts out then pressure stops building, when released the air pressure starts to drop in both the (F) and (R) tanks.
I have attached the motor and air compressor with soap water and found nothing for leaks, also attached the governor mounted on the firewall and no leaks.
I can build pressure to below fan cut off and it will hold pressure, for a couple hours. (enough to sleep for the night with the LAW alarm not going off)
With this same issues I can not do the following when the pressure is below 70psi
Shift Hi/Lo, dump air bags etc.
Someone said it might be the valve in the head, but I have no clue, I am trying to get this fixed asap, and as cheap as possible.
I have been doing most all my repairs to my truck.
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