Glycol is a funny substance. It can find leakpaths that 100% water can't. It has something to do with the glycol's surface tension from what I've been reading. There have been threads documenting the leeching of glycol through propress joints as well as conventional threaded joints.
I'm not sure of glycol's ability to break down joint compound...never gave is a second thought....I'm interested to see the responses of anyone experiencing it.
By nature, glycol is not really friendly to hydronic systems. It effectively "undersizes" circulators because it's thicker than water and increases the head loss of the entire system. Rate of heat transfer is reduced as well, compromising the efficiency of the heating plant.
Try searching some of the heating forums as well as listening to the people here...you're bound to findan answer:thumbsup: