pressure relief valve causing knock

i installed a pegasus 5000 series shower valve in a new shower. soon after knocking was heard after shutting off any valve in the house. the sound was clearly coming from the shower. i opened the wall behind and installed 16" risers to both hot and cold lines. knock disappeared. a few months later the knock is back. i drained the system and recharged, but the knock is persistent. the shower valve has internal shutoffs. i shut them off and the knock was gone. if one side (hot or cold) is off and the other open still no knock. with both open i get the knock immediately. i am starting to think the pressure balancing device is to blame. on this model there is a bore just left of the cold shutoff. it has a washer and a bolt in it. the spec sheets show picture of the valve and this extra bore. they show a "cartridge" in this hole.
ok, to the point...i'm wondering if this extra cartridge is something i need to take a look at, or if it's one of the shutoffs that is to blame, or if it's the central valve.
phew!
thanks
beng

knock

If the valve is working properly and you shut off either the hot or cold valves, no water should come out of the shower head. That is the reason for having the pressure balancing function.

knock

thanks. i understand the reason for the pressure balance valve and at closer inspection i see that the bore to the left of the shutoff is where the pressure balance cartridge is located.
if i shut off either side no water comes out so i guess it works fine.

so what is causing my knock? could it just be a faulty washer on one of the shutoffs? like i said...if i shut the valve down completely the knock disappears immediately leading me to believe that the valve is to blame.

Deb

I would agree that the valve is to blame, but whether it is in the shut-offs or some other part of the valve, is impossible to know.

Deb
The Pipewench