You have a couple of things going on here.
As I see it, you shouldn't see any 'changes' in the water flow when the new bathroom is NOT in use. If the new bath was IN USE at the time you flush or shower in the UPSTAIRS bathrooms, then you should see a change in the pressure.
RE: "... on the same line until the main one going out of the house." I'm not clear what this has to do with 'pressure' in the feeder line. If you're referring to the sewer line, it's moot. It's gravity-fed and that's that (I'm assuming).
RE: "... faucets use to shut off immediately ...", again, I can see this if the basement bathroom is IN USE when you go to use water from a 'down line terminal' (which is an upstairs sink or bath or toilet or whatever.) If the water is NOT in use in the basement, that's moot.
Is the washing machine or dishwashing machine running when you go to use the upstairs faucets??? You could have the lines 'tested' by a Plumber for proper pressure. Or, you could buy a Tester on your own and do some limited testing. I know I keep saying this but if the basement bathroom isn't in use, I guess you could have a 'leak' somewhere. Check to see if your meter is running when all the water in the house is OFF. Others may have more info so stick around for a while.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!
Thanks for the reply - Believe it or not, it helped a lot. My knowledge on plumbing is limited so you eliminated a couple of things. I will check the meter as a leak seems to be a possibility. Other then that, I will call a plumber to test
For what it's worth, have you checked the main water supply valve? I ran into this problem at a friend's house after a plumber installed a shower in the basement. The problem was that the plumber didn't turn the main water valve back to fully open, hence the reduced water pressure. Sometimes, it's the simplest things! Let us know.