Probably not what you want to hear....pump that piston out first when the system is full of fluid, making sure the bore cap side is being held into place .
I used to grind my rim like an animal so mine did not work so well unless mark ground my rim (he has the special touch)
Indeed I am and loving it. The trialtechs would take some "slight" modification to get them to fit in the tnn backings but they do work so well in them.