In laws house, has a 40 yr old septic gravity system. All is well until heavy rains cause a back up into the basement of the house. Only two people living in house. Its gets pumped every two years.

Over the years my father in law has planted trees and installed a pool, to close to the field in my opinion. The trees where taken down last year, and I think when the pool was installed there was some re-grading done that affected one of the legs of the field. Nothing I can do about that.

I think when it rains water is pushing back through the slightly sloped field and the line between house and tank into the basement.

They are elderly and on a fixed income. I am just trying to get it from not backing up into the basement and cause a mess. They can live with being very water conscious during heavy rain periods. The good news is they have a reverse mortgage on the place and when its all over, its going to be somebody else's (bank or mortgage company) problem to sell and not mine.

Could a main line check valve (3" or 4') be installed between the house and the tank to prevent a massive backup and mess? Any suggestions on brands or types?

You can install a check valve on the sewer line but it would be much better to change the grading that allows this to happen in the first place. I generally buy Oatey and Sioux. Where exactly is the water entering, a floor drain or shower?

Could a main line check valve (3" or 4') be installed between the house and the tank to prevent a massive backup and mess? Any suggestions on brands or types?
Yes there are sewer backwater check vales,( see image) They are installed just downstream from the house cleanout. Check them out at: :http://www.nextag.com/sewer-check-valve/search-htm
For a larger selection Google "Sewer Check Valves".. Good luck, Tom

After couple days heavy rainfall I have a problem with my septic field. Backig up to a septic tank. What can I do