Richard, You'll have to forgive me a little lack of knowledge on the 150, but when I had this problem with my 200BF I re-assembled the brush *without* the needle and pressed the trigger. The air came out ok, proving the valve and hose were working correctly. I then re-assembled with a brand new needle. No air came out
The problem was eventually a sludge of acrylic paint inside the two parts of the nozzle assembly, which was blocking the flow of air through a couple of bleed holes in the tip.
I got a selection of Falcon brand pipecleaners (here in the UK at the least) they were the thinnest available and would fit into the needle bearing ok. I dipped a short piece in Iso Al to remove any chance of lint or dried paint clogging, then cleaned the inside of the nozzle parts until they shone. That did it