Mule,

How many valves comprise the manifold? Aloha

There are 6 zone valves on the manifold. I'm hoping I don't have to rebuild the entire manifold with new valves, just because I can't source a diaphragm!

Mule,

Your picture looked like it was not easily feasible to replace one valve at a time. How old is the manifold? Will the other valves start to have problems after you fix the leaking one? Can you live with the leak? If possible build a gravel sump that the water can drain to and leach into the soil. Otherwise it would be easier to replace the whole manifold and configure it to be able to replace one valve at a time. You would have a new manifold with no problems that may surface for a good length of time. Depends on your system's age. You may try removing the diaphragm, check for debris, clean and reassemble. Here is a link to a "How to" for a diaphragm:
http://www.ehow.com/how_6522537_replace-diaphragm-sprinkler-valve.html
There also utube vids as well. JMHO. Aloha

I removed and cleaned the diaphragm, but the leak persisted. I took the old diaphragm to a irrigation supply house here in town. They were able to match it to a replacement, and it now works. Problem is, the replacement diaphragm is no longer in production, so I bought a few extra. My manifold system is a mess. The valves are so close together that you can't cut one out and replace without disecting the entire manifold. So, when the next valve fails (and a diaphragm can't fix it), I'll be replacing the manifold and updating the valves. Is there a particular brand you recommend?

Mule,

I don't have a preference but I am sure others have discussed brands they hate or love in past threads. To me, the cost usually dictates quality in this case. When the time comes, configure your new manifold with the ability to replace each valve separately. If you want to know how to do that let me know. Glad to read you were successful. If you weren't, I would replace the spring with a new diaphragm to ensure a tight seal of the diaphragm (these are usually included with a diaphragm repair kit). Aloha

I have a Rain Jet system that was installed in 1978. I need a 1 inch T with 1/2 riser connection port. Can't find one (or More) anywhere. Please help.

if you cannot locate the proper T at a sprinkler supply house or a store like Home depot you may have to resort to a 1" x 1/2" reducing bushing. they make them in both threaded or slip x thread so you should be able to adapt things to your needs.