One of the key problems with older plastics especially those exposed to hot
glycol/water mixtures is that the plasticizers get leached out of the
polymer. The result is at least embrittlement and often phase separation of
inorganic fillers which had been added to the original chemical "alloy" to
provide hardness and rigidity (sometimes swelling reduction). The net result
is often fractures and crumbling under stress. In general adhesives work
poorly here because the inorganic filler coats (actually segregates out on)
the fracture surface. The adhesive then tries to bond to the equivalent of a
thin film of chalk. Repair requires another polymer that can encase the
original part using it as a mold. Fiberglass resins and specially formulated
silicones work well, but amines and epoxies will fare poorly against the hot
water/glycol mix. A copper brazed custom part is easy to fabricate,
Electrolysis concerns can be mitigated by acid cleaning, rinsing in pure or
distilled water three times, drying then immersion in a marine Teflon coating
(paint), followed by a long term (few hours) cure on in an oven at 250F.

Frank Grunthaner