I have Vault hubs on my dual axle trailer. The trailer is about 4 years old. I have looked at the back of the hubs where the rear seal is and 3 of the 4 hubs are dry with no grease out the back of the hub rear seal. The 4th has a tiny bit of grease leaked out the back. It says on UFP website that "some leakage is to lubricate the rear seal". My question is, how much leakage is acceptable?