I have a daf lf 2007, went to start truck last week and the red air low warning won’t go off. Have changed air dryer and a new canister issue still the same. When you go in air system on computer screen it shows only 1 of the 2 tanks filling fully, the 1st one only get a qtr full and won’t go no higher. Checked compressor seems to be building air ok. No obvious signs of air leaks.
Any idea what it could be?
it there isnt an air leak and the compressor is working blocked pipe
Manual tank drain? Could be full of gunge
Is the pressure sensor for that side knackered?
Do you know where it’s located? I assumed the sensors where in air dryer unit?
Will try and look for blockage and drain tanks. 
Have a watch of this guy on YT :
