Recently I replaced the original elbow (40+ yrs old) on my upstairs sink because the plug had rusted out. After doing so the sink worked great - unfortunately the next day when the upstairs shower or the kitchen sink was used it backs up into the main floor bathtub. I have tried plunging while sealing the overflow valve and pouring boiling water down the drain. Can you Help?

speedball1

You have a clog downstream from the bathtub. The drain line must be snaked and cleared. You can go in through the 1st floor bathroon lavatory vent on the roof or you may remove the tub overflow plate and pull out the brass stopper and go down past the trap. Your choice, but one way or the other that drain line has to be snaked out. good luck, Tom