The reduced diameter of the piston allows the front of the piston to go past the "step" in the tee.
You must have a difference in area for the piston to actuate. ie. you cannot use a 3/4" porting, if you house your piston in a 3/4" tee. *
Going to 1/2" porting (extend the barrel about half way into the tee) will give you the differential you require.
You can buy a reducing bushing to accomplish this without having to re-do everything.
* It can be made to work by using a burst disk downstream of the piston valve and pressurizing that area, but that's another story for another day.
