the valve in the filter is a non return valve. fitted so the oil doesn't drain back when standing, therefore giving you a pre primed system on start up. the filter doesn't need a pressure relief valve as it's upstream of the one fitted in the pump so would be redundant.
neil.
Well, if you look it up, you get articles that say there IS a bypass valve in modern filters. I believe they are confusing the two. The WIX I used had no bypass valve at all and barely had an anti drain back valve.