sand point, air in hot water lines only

just installed a new sand point, this site helped tremendously. Lots of water but i got the same thing happening as the last sand point. I have air in the hot water lines only. Is this because i never ran the lines properly. If I run cold water no problem, but contrary for hot water. Does presure build up in a hot water tank for this to happen. once it runs for a while no problem.

I also used normal union couplers, only have two on the length for I used a 10 ft section. The off shore couplers I bought from reputable canadian supplier shall I add threaded in the pipe on a slant. Should i be ok with these 1 1/4" union couplers, I used pink teflon.