Toilet flush causing main trap level to rise

[Casey Gray]

Hello all,

I am having a potential issue with the water level in trap of my main line. When the toilet is flushed, the level will rise by an inch. When the toilet is flushed repeatedly, the level will rise by several inches. When using other fixtures (dishwasher, faucets, etc. the water level does not rise.
I am selling my house and this was discovered by the buyer's inspector. For all I know, this is normal, but I wanted to check in with some knowledgable people before I have a plumber take a look.
Some other info: The house is 80 years old. The line going from the house to the street was scoped 3 years ago and nothing unusual was found. Image included
Thanks a ton for any input.

 

[Slomoola]

Tree roots, no vent on the drain line, poor flushing Toto toilet and that undersized 2" drain looks like somethings to check on.

 

[Sponsor]

 

[WorthFlorida]

A home that old usually had a cast iron trap before leaving the house. They are no longer needed. With two removable caps it should be possible to open them and check for any debris. It is odd that running water elsewhere doesn't affect it. Does this old house have old toilets that flushed up to 5 gallons per fill? It just might be overwhelming the pipe. At a faucet it may be 3 gallons per minute such as at the utility sink shown in the picture. What happens when you run the sink next to it?

 

[Jadnashua]

A toilet dumps a lot of water quickly. A sink does not. If water doesn't back up in the line, I don't think it's a problem, but I'm not a pro.