Psi question

Do the Volvos have seperate gauges for both systems or are they a two needle single gauge? Gonna be an air leak unless the primary air gauge is going bad and reading wrong. Mix some Soap and water in a spray bottle. Spray everything down on the primary side to inspect for leak including the gauge in the dash.unless there’s a newfangled valve or something in the system not allowing it to air all the way up.

 

http://volvodocs.hhrvresource.com/40-59/PV776-TSP194846.pdf Read the part about the 'air brake system.'

 

Since they are separate can you switch the hoses or electrical connectors on the back of the gauges and see if the problem follows the swap? If it does then you have a system/sender problem but if it dosent then you have a gauge problem.

 

Ever pull the drain valves open? I'd start there. After building pressure, shut the truck off and drain the tanks.

 

This^^^