Abstract: A hub cap for a vehicle wheel, such as a boat trailer, which hub cap retains grease and blocks water flow into the wheel bearing while providing for automatic expulsion of excess grease when pressure increases within the hub, such as is caused by heating during running. A hub cap with a housing for mounting on a vehicle hub and including a fixed central member within the housing having a boss projecting into the outer zone and carrying a grease fitting for injecting grease into the inner zone. A seal is slidably mounted within the housing and engages the boss in sealing engagement, closing openings in the central member blocking grease flow from the inner zone to the outer zone. A spring urges the seal into sealing engagement, while an increase of pressure within the inner zone moves the seal out of sealing engagement against the spring force.