I appear to have developed a tiny leak on the underside of my expansion tank - under pressure only and very small 2 to 5 quarter size drops when stopped over an hour. It does not appear to be a hose, are there any typical failure locations under there, it does not appear to be the seam either.

 

they all leak eventually heck I even had the metal tank and the dam thing leaked on the outlet of the small nipple

rock auto is your friend

 

The smaller nipple is a known failure point.

 

Found the leak that U/V dye just keeps paying for itself. Bottom rear by the rectangular bracket literally a fingernail long crack. Now I just need to order a new tank.

 

Just had this happen. Mine started leaking on the bottom nipple. Check out FCP Euro as they sell Mahle expansion tanks. It's a nice quality item as the plastic seems better quality than ones I previously picked up off another sites.

 

I'm a fan of the Mahle tank too. Been running one for a while and all good.