try some plastic weld. it is an epoxy type stuff and works pretty well on plastics. I used it to seal a radiator coolant bottle in my brothers rover and it held up quite well for a couple of years . and this was a high pressure bottle...

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U