If there is a pressure differential the check valves will leak and/or lift. If you want a tight shut off isolation valves are required.

I would say it even stronger, RobinHardy--the main characteristic of the valves on a recip is that they operate fast and can tolerate a very large number of very fast cycles, not that they hold significantly well. If suction pressure exceeds discharge pressure of a stopped recip then gas will flow through a recip just like the compressor wasn't there at all. If discharge is higher then most of the time you will get significant flow from discharge towards suction.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

It surprises me sometimes how people think that Non return valves and control valves are as good as isolation valves. They aren't and whilst flow through a pump or compressor isn't common, can easily occur, even for small injection pumps if the system is shut in and then cools down - happened to me once and overdosed a section of fluid by >1000 times.... Actuated isolation valves are your only truly reliable solution.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way