Polyethylene and polypropylene are very difficult to bond. This stuff has an activator to prepare the surface, and might be worth a shot. I haven't tried it myself, but I found it when I was toying with the idea of making a pump out of a big syringe, like the one for backflushing the Sawyer.

http://www.loctiteproducts.com/p/sg_plstc/overview/Loctite-Plastics-Bonding-System.htm
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