Oh, I'm seeing the problem. I wasn't envisioning the issue right. If there is already
PVC pipe into the bulkhead and plumbed into the system, you will need to use either silicone or
PVC primer to 'glue' it into place. Them popping out is when water gets under the pieces you are trying to add in to make the overflow a durso, when normally this would have already been accomplished as one solid piece of
PVC during the original plumbing of the system.