The initial material cost differences between copper and plastics are significant. A article comparing costs for different plumbing pipes can be found here: http//:www.southce.org/ajwhelton. A US government funded study to investigate what chemicals leach out of American plastic plumbing pipe is also underway. Go to that same web page and type in 'plastic pipe'. There has been quite a bit of work done in Europe but American materials have not been equally publicly scrutinized. Some of the study data has already been released and testing included actual buildings and testing of new pipes purchased at local building supply stores. Here is a link that might be useful: Informational research website

Though you wrote that everything used was brass and copper, sometimes the tub spout nipple used is steel. SOMETIMES a tell-tale is if you hold the tub spout diverter up or flip the diverter lever to "shower", then turn the water on, is the water out of the shower head mostly clean? THEN divert the water to the tub spout and see if it's more discolored coming out of the tub spout. As to the valve itself being at fault, it'd be idiotic for a company to use steel in their valve. But anything is possible. If you have a any photos of the plumbing work before the walls were closed up, give them a look-over and check out all fittings, see if there's any iron/steel fittings in there. I know, I know. I'm just saying, you know? That issue addressed...most of what follows would be "whole house" symptoms versus just a singular plumbing run pumping out dirty water, or a singular fixture within the house that's pumping out dirty water. Any galvanized or iron pipe anywhere else in the house? If there's no galvanized/iron plumbing anywhere else in the house, then it could be sediment stirred up in your hot water heater, or it can be iron bacteria in your well. These can go unnoticed during normal use, but if a plumbing system gets shut down for a while they can show when the water again starts to flow. For sediment I'll usually drain, flush, and refill the hot water tank. Then remove any faucet aerators or screens or shower heads, and flush out all plumbing lines in the house. If it's iron bacteria, that's a whole new ball o' wax. If you have a rust-colored slime or coating on the inside of your toilet tank (not the bowl, but the holding tank) that's a good indication of iron bacteria. You can sometimes get an indication of the source if it's more "hot water centric" versus cold water centric. If hot water, it can be sediment or sulphur bacteria in the hot water tank that's been stirred up. Now with all that written? My first point of attack would be an errant piece of iron used when the bathroom was plumbed, I'd start with the tub spout nipple.

Starving of other fixtures is entirely dependent upon how much water is exiting each faucet. The size of the line to your manifold will have a much smaller effect than the number of fixtures you are using at the same time. That said - follow code as it is there for a reason.

We were going to do a couple with a new piece of PEX tubing and a new fiitting at the valve, but very limited space to work with and when we couldn't get it loose from the valve, we did the cap instead. It is also possible that the parts we found wouldn't work with our PEX. We need to get the whole house replumbed it appears as ours from research seems to be from Kitec.