How do I fix a sprinkler valve that leaks instead of popping up the sprinkler heads?
First we need a Manufacturer and possibly if you can find it, a model number. Irrigation control valves like this have two parts, a diaphragm operated water control valve and a vacuum breaker. Typical Rainbird system shown here.



The control valve uses an electrically controlled needle valve that operates the main diaphram valve through servo action (small force controls large force). Cross section to give an idea how this works. When the needle is seated, full water pressure operates on the upper side of the diaphragm, shutting water flow off. When the needle opens, the pinhole in the diaphragm is smaller than the needle opening, pressure dumps and the diaphragm opens.



Where your leak is occurring is in the vacuum breaker (right hand side below). This device operates to prevent water siphoning back into the water line through the control valve if water pressure drops. It is essentially a check valve with one side open to the atmosphere that closes under pressure and if the parts are damaged through wear, foreign object stuck in the seat, corroded in place, etc. becomes as you've found, a large volume leak under pressure.

This is where having all that manufacturing information is important as most of these can be repaired if you can find the repair kits. Otherwise, it's complete replacement time.

Exploded diagram of the above example Rainbird unit less the solenoid control.



Champion Classic Brass 3/4" Automatic Actuator and Anti-Siphon with Union

Manufacturer is Arrowhead-Champion http://www.championirrigation.com

I've used Irrigation Direct for other product, their replacement parts page. There are other businesses listed. If you have a large irrigation supply anywhere near, they probably carry the product. Arrowhead is a well known manufacturer of brass plumbing fittings.

The Champion product has been around in manual systems since the 1930's. To get the Vacuum Breaker loose, you might need gentle heat on the body. Keep it on the breaker side.



Breakdown of the original Manual Valve and Anti-Siphon with Union



And the bits in the Anti-Siphon Valve that are giving you grief