Repairing a sprinkler valve manifold can be difficult. Sometimes you have to extract the complete valve manifold and start over by re-plumbing in a brand new one.
Here is a picture of a before and in the process of installing a new control valve manifold. As you can see in the picture the old valves had many patches and repairs made during the years up to this point. Control valve manifolds usually last 25 to 35 years depending on the material type used and quality of the installation itself. Unfortunately I have seen through the new building boom some very expensive brand new homes in which their sprinkler valve manifold is needing to be replaced after only 3 to 5 years because of low quality craftsmanship during the installation. I first approach a problem on a valve manifold to see if there can be a repair made such as replace a component or something adjusted or a component replaced before resulting to having to extract the complete manifold and replace all of it. Unfortunately sometimes because of the original installation there is no other way but to replace the entire unit if something breaks or starts leaking.