Why would the anti-freeze spigot have water coming out the top cap when the hose is attached and the water is on?

Hi Dito

If the cap is leaking then the VACUUM BREAKER is defective and in most cases can be repaired by purchasing a vacuum breaker repair kit from your local plumbing supply store or online. You will need an exact match so the brand name of the antifreeze faucet will be important here, OK?

Below is a video showing how to repair a defective vacuum breaker:

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Let me know if you need anything else...

Mark