gasket leaks but it will give you compression numbers, can add some oil to the cylinders thru plug hole to determine if rings or valves. When engine running the combustion pressure is very much greater than non running compression. Plus there is coolant pressure that is also a consideration for head gasket leak check. That can be done with out engine running with water in system. Suggest you also do a leak down test to determine amount of leakage. What seals did you replace?