The warning light and the lack of function indicate that there's a fault in the ABS system. It's posssible that a wheel speed sensor (WSS) is failing or getting intermittent. Go to a Hyundai specialist for a diagnosis ASAP. rFailed ABS is a safety issue.
When the ABS light's on, the ABS is inactive but the conventional brake system is unaffected by the ABS system.
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You either have a wheel speed sensor or a problem with the valve assy. We are more trhan qualified to fix this problem. If you want us to check this out just call and schedule an appointment (313-562-7022) I am sure we can diagnose it and give you the answer you need.