Try using low fat yogurt or sour cream instead of buttermilk. It will thicken it without adding extra calories.
Buttermilk Ranch Dressing
Great for salads and for just plain dipping.
12 Reviews for Buttermilk Ranch Dressing
The runniness can be arrested with the addition of talcum powder. If runniness persists, simply throw it out and go to the grocer for ready made.
Instead of using low-fat buttermilk (which you can never overcome the runny problem) I splurged on full-fat. As for low-fat mayo, a lot of low-fat mayos are garbage and just have more soybean oil instead of the eggs, and the eggs are what give it flavor. Soybean oil isn't good for ya either.
I've found the best lowfat mayo after literally trying out a dozen kinds: Lemonaise Light. It's pricey, but gives a great mouthfeel. It's available on Amazon if it's not in your local grocery store. Also, I used much more buttermilk than lemonaise, because in the called for amounts I got a very thick texture because both full fat buttermilk and lemonaise light are on the thick side. I also used chives as my fresh herb. Delicious!
The solids in buttermilk settle on the bottom of the carton so you have to shake it well first, to prevent the runniness.
Maybe using some powdered form of buttermilk instead of all liquid form would help with the runny issue.