I have taken just taken apart my shower valve to try and flush the pipes and clean it out as I am getting very low water pressure (elsewhere in the house and in the bathroom, I have no problems).

It's an American Standard. I don't know the model number (was already in place when moved in) but it looks like this: http://images.lowes.com/product/012611/012611319143.jpg

At the back of the pressure balance unit are two small green parts (as pictured below). I thought that they might be flow limiters and so I reassembled the unit without them. Now I'm wondering if they're check valves or something that should be re-installed.

What are these?

I have searched thoroughly on the American Standard website for a parts diagram and cannot find one.

Thanks for your help.