Your pressure is low. Did you try out the liquid sealants?
I don't know what is ths ealant for this application, never been in plastics, but the liquid sealanta have the property of filling up the gaps, and then again you don't have to put that much torque so as th crush the threads.
BTW: the torque to crush the threads are likely to seal better, since the softer material will flow and fill up the hap, unless your torque is washing them out.
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