I am on a well water system with a fairly new expansion tank and all the
water lines are copper, inside the house. Whenever I do anything that
requires turning off the water and partially or fully draining the system,
such as installing a shutoff valve somewhere, when I turn the water back on
it comes out very dark for a few minutes then eventually clears. If I fill
a glass while it's doing this, there's all sorts of black crud floating in
the glass. Wondering if it's crud from the water heater, or from what's
inside the pipes that breaks loose when the pressure is first turned on.
It spits air and water the first few minutes, that's why I am thinking it's
just normal crud build up that breaks loose while it's spitting air. Any
ideas?

thanks