It is normally the bottles themselves. When you do the clip up, it squeezes the bottle connection slightly, allowing it to continue weeping.

Try and find a metal insert they you can slide into the connection (must be a snug fit) and then try connecting it back together. This should prevent the bottle connection from crushing and therefore prevent the weep.

It is a common problem with expansion bottles for coolant systems. Most manufacturers now put a small metal insert into the hose connections, especially on the smaller-bore return ones. Same principle would apply to PAS bottles though.