Stemco 359-6125 Hub Cap Plugs. To relieve vacuum, the Sentinel interior check valve lifts slightly, drawing air through the water-resistant expanded PTFE fabric filter into the wheel end, while the duck-bill check valve remains closed, blocking dirt, grime and other contaminants from entering.. To relieve pressure, the Sentinel duck-billed pressure relief valve opens, allowing pressure to vent out, while the umbrella check valve remains closed, stopping lubricant from leaking out and contaminants from flowing in.
