2007 LC78 - Weeping Sub-tank

[Alloveroz]

There appears to be a very small leak on the Long Ranger 180 sub-tank, indicated by red dust sticking to the tank, not enough to produce a pool on the shed floor. It appears to be coming from the top of the tank. There are no signs of leakage round the filling system or the sender mounting.

Are there any connections on top of the tank?

Has anyone else had this problem?