MG Car Repair: MG Oli filters One way valve

Question
Thank you Barrie for solving a mystery!- my 74 MGB oil gauge usually takes a few seconds to register pressure and this always caused me to wonder why, but I assumed a sticky gauge. Other times it showed pressure immediately, which now I know meant I had installed the correct filter.   

Q: how do I know the filter has a one way valve? can you see it upon inspection? I have never heard this mentioned on any other blog.
thanks Paul

Answer
Sorry for the delay, I just moved house.

If there is a long delay between starting a cold engine and a reading on the oil pressure gauge, then it is probable that the system is draining due to the lack of a non-return valve inside the oil filter element.  I always buy my oil filters from a reputable supplier such as Moss.