the photo of the pipe in post #12 is definitely HDPE pipe. zooming in on the photo in post #9 it looks like you have a close header but I can't tell if it was site built or pre-fabricated. Site built would mean lots of joints between short pipe sections and T fittings so lots of opportunities for leaks because there are lots of fusion connections. I would tend to think pre-fabricated headers would be more immune to a leak but anything can happen. Fusion connections aren't rocket science but there is a process to follow - you hold the pipe and the fitting it is going to connect into against a hot iron at the same time heating/melting the outside of the pipe and inside of the fitting. Then you take them off the iron and press the pipe and fitting together allowing it to cool. Its like connecting pvc pipe but instead of using primer and cement you are using heat via the iron. If it was my system, I think I would have them re-pressurize the loop first without the stop leak and see if it stays pressurized. If it stays pressurized, then that would indicate something along the lines of what ChrisJ was talking about - blockage in a section of the loop that was never originally purged of air that recently became unblocked adding a bunch of air to the overall loop. If it doesn't stay pressurized, that would indicate a leak somewhere in the system outside (I'm assuming you would see the leak if it was in the pipe in your basement). Then re-pressurize with stop leak and see if that allows it to stay pressurized (ie maybe the leak is small enough to get clogged by the stop leak). If that works, you may want to consider converting to a non-pressurized pump system so to keep internal system pressure as low as possible at the now clogged leak point. If the stop leak doesn't work, I would probably then go and dig up the manifold, tear it apart, and separately pressurize each section of pipe and see what holds pressure and what doesn't attempting to isolate the problem to one of the slinky loops, the manifold, or the pipe leading to/from the house.