Blowout preventers, fluid pressure systems and portions thereof may be tested for leaks by calculating an Apparent Compressibility Factor which is determined during initial pressurization of the system to the test pressure. The equivalent decay rate is calculated by measuring the amount of intensifying fluid required to be added to maintain the system at the test pressure level. The equivalent decay rate in psi per minute is then compared to the acceptable decay rate for the pressure level of the test to determine if the system passed the test.