I have that system too. Agree - the guages aren't that great, and first thing to do is check for actual operation. Also, do you have normal returns in your deep end that turn on and off with the 6 port valve - you may not see in floor heads popping up, but it the water has to be returning somewhere and its probably those deep end returns.

If thats the case,your 6 port valve may need to be replaced. They only last ~5 years on average. They cost something like ~$125 to replace.

You can order replacement pressure gauges too from Paramount directly.