Vacuum Head problem??

[JMerk]

My sprinkler system decided not to work yesterday, it had worked fine for the last two years, no problems. I ran it in manual, nothing. I checked the rain sensor and it seems to be fine. I eventually got the system to run by turning the valve after the above ground "Vacuum or pressure" head on and off. The sprinkler system ran afterwards. This mornign I ran the system in manual again to check and nothing. I turned the valve on and off and it started fine. When the valve is turned open again I can hear water run through, is the vacumm head sticking or do I have a problem with a leak in the ground letting pressure bleed off?

Thanks

[upper]

How many valves total and do they all not work? Upper