I have a drainage pipe from a downspout that runs to the street right behind my equipment pad. I added a wye reducer to plumb my vacuum to waste line so it can easily go out to the street. This wye is roughly 18" from my multiport valve so in the very rare occurrence that we get heavy rain and my drainage gets backed up, storm water could potentially back up to the wye and into the waste line port. Would this require a check valve, or nothing can back flow in as long as the multiport valve is NOT set to waste?