if sure the air dryer is ok check air is getting to the park brake relay valve ,and that it is getting a signal from the handbrake lever and air is getting to the brake chambers ..i would also check the condition of the air dryer cartridge ,jus in case the compressor has a problem and is causing the air dryer to fail ...port 25 feeds the hand brake and has a NRV that sticks.you can also remove the bottom bowl ,remove the 2 plastic filter plates and put the bowl back on
norb, January 2011