Like Thomas said, you need to lube the o-rings in the brakes, but go to a store that sells pool supplies and buy a small tube or bottle of silicone o-ring lube.

If you lube the o-rings and still have a problem (and everything else in the system is OK), you can try placing an airline restrictor in the airline to both brakes (1 on each side). I usually place the restrictors just a couple of inches from the brake hub. This slows the activation of the brakes but does still allow full air pressure to them.