If the actuator is set for 9psi, then that's all you'll see by removing the amal valve from the equation - the amal valve bleeds off to give the increased boost levels. They are serviceable though:

http://www.forgemotorsport.co.uk/con...t&catalog=0019

If the spring has weakened, it could be being overcome by boost pressure at higher levels. Could be something amiss elsewhere like abdr500 says, do you have a dump valve fitted as that could be leaking. Any strange noises?