I'm not a proffesional, but!!
Did you check the bleed screws on your valves? Maybe when you (or a company) were winterizing the system. Make sure they are all "snuggly" tightened. Other than that suggestion, thats all I can think of. It would be kind of weird for all 4 valves to go bad at once.
I'm no expert..........YET!
I just like to suggest things and learn...
See what the pro's have to say first.