Flushing a Moen Posi-Temp valve

I am remodeling two bathrooms in my house. I just installed a Moen posi-temp (with stops) valve in one of them. The installation instructions recommended removing the cartridge and flushing the valve before running water with the cartridge installed. Even though I had the stop tube in place, it was still difficult to get the water to just go in the tub and prevent the water from leaking around the valve and into the wall behind the valve. I have another valve to install in the other bathroom. Any suggestions as to how to accomplish this without leaking water all over the place?

leave it alone

Moen Posi Temp are mean to fool with
even when they are new...

if you must do that I guess you could find a slightly larger
pipe than the shaft coming out of the wall
like a 1 1/2 PVC tail piece to funnel the water
into the tub....

I would probably not mess with it if it were mine..