2005 m2106 freightliner

Hey first time posting looking for some
Advise. Have a 2005 m2106 with a mbe900. Truck won’t build any pressure in the air tank. It has a single tank dual chamber. Took supply line to drier off and compressor is pumping air. I then disconnected the output line from the drier to the tank still have air output. I then wanted to make sure the compressor wasn’t weak I took the output line and put it in a spare single chamber tank I have for a trailer that all ports were blocked off and had gauge installed. Reached pressure no problem. So I knew everything from the input of the tank to the compressor was good. When I start the truck hooked to the original air tank I crawled a round and can not hear an air leak anywhere. Im fairly new to the trade so im kinda stumped where to go next. It is obviously a very large leak somewhere or a bad tank? Any help would be greatly appreciated.