The fuel return line has a tank breather incorporated into it by using a t piece just before it enters the tank. While not ideal it has worked for many years without issue. Recently fuel started to backup in the breather. Engine runs fine , fuel return and breather clear. Opening the filler cap fixes the issue of course and there are plenty of fixes, its just confusing as why its doing it now. Breather is vertical and ends at highest point. Yanmar 3ym30 and the return flow is a steady stream back into tank. Any ideas welcome. Many thanks