Response to Hello Tony.  The purpose of the Outlet Elbow O-ring on the Hayward Filter series is to provide a seal between the Stand Pipe and Filter Manifold Assemblies.  Filtered water passes through the inside of the Stand Pipe.  The water outside the Stand Pipe is unfiltered.  In the case of Hayward DE Filters, Stand Pipe O-ring failure allows DE to circulate back into the pool.  The Stand Pipe O-Ring should be lubricated regularly.  The inside of the Manifold connection sleeve should be kept free of build-up from previous O-rings and lubricant.  This type of build-up can reduce the life span of replacements.  Silicone or Teflon based lubricants are acceptable in this application.  Petroleum-based lubricants are not. 

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