When using the shower or washing machine during filling, what would be the reason the pressure drops such as when the toilet is flushed? In my mind I would think the water lines are old and full of buildup thereby either needing to be descaled or replacing.

Not uncommon. Volume flow restriction is the most likely culprit. Are you on a well or city water? Make sure all of the valves on the supply side are fully open. Shower riser is 1/2" typically, flushing toilet drops pressure and subsequent flow. Never "descaled" piping.

Not uncommon. Volume flow restriction is the most likely culprit. Are you on a well or city water? Make sure all of the valves on the supply side are fully open. Shower riser is 1/2" typically, flushing toilet drops pressure and subsequent flow. Never "descaled" piping.

Its on city water but it is an older house

What material are you water pipes? If galvanized, they are most likely plugged up beyond repair, so replacing would be the only real fix. Also, if your home is really old, you may have a lead service line. I do, and I will be replacing it this fall after ten years of dealing with not allowing anyone to flush a toilet while Im in the shower. I even tried a booster pump, but the line is so plugged the pump can't take in enough volume to supply adequate water to the fixtures.

I can sure relate to that. rwjohnson61 nailed it! You, indeed, have a volume problem. Let me explain. Older homes like yours and mine were piped with galvanized iron pipes. Over the years Iron oxide, (rust) builds up in the popes to the point that you end up with a 3/4
pipe with a opening the size of a pencil. No volume = no pressure. I just got through replacing my galvanized water pipes with PVC to get the volume and pressure I needed.
Want more pressure? Get three estimates on a repipe job and go with the one that you're most comfortable with. Good luck, Tom

Before you tear into any repiping you should check your psi. Test at the garden spigot and a simple pressure gauge adated to the bib. Then test your volume or flow rate by placing a 5 gallon bucket under the tub spout and time how long it takes to fill 5 gallons, back to you

Hey Bob,
There's only one answer to this,
When using the shower or washing machine during filling, what would be the reason the pressure drops such as when the toilet is flushed? Older home. Galvanized pipes. No volume. If you have another solution then a repipe job now would be a great time to trot it out, rwjohnson61 if you want to check the house PSI as Bob suggested then you'll need this,(see image) Cheers, Tom