Use 5 minute epoxy or puddy epoxy to seal the leak after drying it if there is a crack.

If the leak is at a gasket or seal, then try to tighten or remove and replace the defective part.

I suggtes check this link about TROUBLESHOOTING guide to solve it (page 10 and next), sound like a install problem, its a vent to allow air in. If its leaking it may be faulty but it sounds like your saniflo is in need of a service.

Hope helps.

Hello,

This are likely solution to stop the water leakage below

Clean tank ball and drain seat thoroughly. If leak persists, replace the worn tank ball or flapper with a Fluidmaster Bull's Eye Flapper Model 43 or 48, Bull's Eye Super Flapper Model 43/48 or a Flusher Fixer Kit replacement seat for damaged brass, plastic or ceramic flush valve drain seats.

Make certain the black vinyl refill tube is attached to the top of the overflow pipe with the angle adapter (see illustration). If the angle adapter does not fit onto the overflow pipe. Fluidmaster offers a Special Refill Clamp Model 43/48
Reset to a lower level by using the water level adjustment clip (see illustration) located on the link of the valve. Squeeze the two ends of the metal clip together, then slide clip down the link.

Replace the entire overflow pipe/flush valve assembly.

Solution 2

Shut off the water supply and remove the Valve cap (Model 43 or 48: lift arm first) by rotating 1/8 turn counterclockwise. Cover the opening with an inverted cup, and turn water on and off a few times. This should flush and remove water line debris. Turn cap over and clean seal surface by rinsing under running water. Replace cap (43/48: lift arm first) by rotating clockwise. Turn on water. In the case of well water supplies, the best long-term solution is a filter.

Shut off main water inlet valve to the house. Turn on all water outlet valves inside and outside the houise, then flush all toilets. Allow outlets to drain and pipes to fill with air. Then turn on main water inlet valve and fill and shut off inside and outside outlet valves. Remove toilet valve caps (43/48: lift arm first) by rotating 1/8 turn counterclockwise. Hold a container over the uncapped valve and turn water supply on and off a few times to clear debris. Replace top by engaging lugs and rotating 1/8 turn clockwise.

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