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.

My first guess would be the air governor. You can usually hear or feel them leaking air. They only cost about $12 and only take a few minutes to change out.

I had the same problem with my Cummins and it was a air filter for the turbo. It's a little spin on filter that sits under the power steering resovoir. Mine had a hole where it had rusted through, I didn't even know there was a filter there. When I soaped everything down I never found the leak because it was on the bottom of the filter.

ck your cluch-fan, when the eng/key off it turns the valve off so you wont hear the air leak if its bad. build your air up the turn the eng off turn key back to run/eng off and ck the cluch-fan for air-leak.

Where would that be on the truck?

Where would that be on the truck?

Are we talking about the air dryer filter? or something else and where is it located on the truck?

The air governor usually sits on top or next to the air compressor. It is about 6" long. You should see air hoses coming from the air compressor going to the air governor. You should be able to see what it looks like by googling "air compressor governor." They are usually available at most dealers and truck parts stores.

No it's not the air drier filter. Mine sat down by the drivers side frame rail under the power steering resovoir. It has a air line running to the filter then a steel braided line comes out of it to a turbo controller (small aluminum block) that sits on top of the valve cover.

I'm starting to see some truck built with the governor attached to the air dryer as crazy as that sounds.