Engine

70 pounds is quite low for the mileage you listed but of equal importance is they must all be about the same. 150 pounds is typical, depending on which engine you have. 125 is acceptable. On a few engines 160 or 170 is normal.

You can also do a cylinder leakage test to determine why the compression is low. That will indicate a leaking exhaust valve, intake valve, cylinder head gasket, or piston rings. Many auto parts stores will have the tools to borrow or rent. You need compressed air to do the test.