We just replaced the tub/shower in a full bathroom renovation. Original supply lines were copper, and we never had this problem before. Supply lines were cut and redone with pex. Elbows are brass NPT/pex fittings. The shower nipple is brass; tub spout tube is copper. The new valve is a Moentrol single handle valve.

When the faucet is turned on, we get a couple seconds of brownish water. This occurs either at the shower or the tub (although the tub water looks darker).

Since it's coming out of both, we're suspecting the valve. Is that likely?

What could this be??? (And, how can we fix it?)

Thank you!