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.