Two mysterious plumbing problems

[mikef]

I am helping my brother out on a 2nd floor bathroom remodel on a colonial buillt in the 30's and have run into two very strange problems.
The first problem is after shutting off the water and cutting out and capping the old copper water lines, there is now very little water hot water pressure in the other 2nd floor bath. The supply lines for the sink were left in place and when I tried opening the hot water valve to run water through the new PVC waste plumbing I hear a strange noise once the valve opens to a certain point. Is it possible there is air trapped in the system causing both of these issues?

The second issue is after cutting out the old galvanized waste plumbing and replacing it with PVC his 3rd floor bathroom (not directly above) sink no longer drains. The trap is clean and it appears to be on a separate waste line. There are two vertical stack lines coming down into the basement. We did not alter any plumbing just simply replaced what was there and everything is pitched properly and drains well in the new bath. Any chance we caused this issue or is this most likely a coincidence?

Sorry to be long winded... just trying to be as detailed as possible.
TIA - Mike

 

[Terry]

On the hot water supply, have you tried to see how much water comes from the shutoff itself?
By taking a bucket, and a flex connector, you can see how well the water goes into the bucket.

Sometimes, the shutoff will be blocked.

On the upstairs drain, is the line vented?
What sizing did you use, and did you use the proper sweeps on the bends?