well when u do a compression check u run the car for a little to see if the needle goes up and falls with leaking compression, and then u do a leak down test by putting a tablespoon of oil into that particular cylinder to see if it regains any resistnace to loosing pressure, if after putting in oil it slows then its most often ur rings if it still leaks out at the same rate its most likely blown headgasket, or leaking valve seals. normally the cylinder has some compression but it can not hold it for any length of time which will not add much power at all, ive ran an engine at 40 lbs of compression on all 4 cylinders b4