Problem blowing out sprinkler

I have a small 4-zone system at my house. My property is about 100 feet deep and 50 feet wide so the sprinkler system does not really cover much land. The system has a Wilkins Model 420 Pressure Vacuum Breaker right where the main water line comes out of the house before it goes to the four zone valves. There is a drain plug in the main water line outside the house that sits right below the Wilkins breaker. This drain plug is where the pressure tap is connected to blow out the system.
I have a Sears 5.5 HP 25 gallon compressor that is rated at 8.6 CFM @ 40 PSI and 6.4 CFM @ 90 PSI. I have 50 feet of 3/8 copper tubing in a coil at the output of my compressor going to a water filter and a quick release fitting. I ran 75 feet of " rubber air hose (after the 50-ft copper coil) out to the drain plug below my Wilkins breaker. I filled my compressor, turned on one zone of my sprinkler and set turned my compressor regulator to 50 PSI of output. I heard a lot of "gurgling" at the Wilkins breaker but my sprinkler heads did not pop up. I raised the pressure a bit to 70 PSI but the sprinkler heads still did not pop up. I had a similar problem last year but after a while of experimenting I got the sprinkler heads to pop at 50 PSI and the "gurgling" went away. I am not sure how I got it to work last year but I cannot get it to work again this year.
Does anyone have any ideas what the problem might be? Is it possible that my compressor is not flowing enough CFM? Would it help if I pulled my compressor out of garage and put it near the sprinkler drain plug and used a much shorter rubber airline to hook up my pressure tap?
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Computer Prog wrote:

I have a similar compressor and have also seen problems like this sometimes when blowing out my system. And I've also seen problems where water mysteriously sprays out of the backflow preventer during Spring start up.
I'm not an expert on what's inside the backflow preventers, but essentially it relies on a moving plastic object being pushed by water to close a passage. I think the problem is that air has a lot less mass, and the widget may not move as easy with air as it would getting hit with water. So, I think it must get stuck mid-way, with the air continuing to go around it. This is probably less of an issue with a compressor that can deliver more CFM. The ones we have are marginal, though they will do the job.
I'd try the following:
1 - Go back to running water through it again. This should get the valve operating properly again. Then you can try air again.
2 - Rap the backflow valve with a hammer to try to free it up without air on it, then apply air and rap again if still misbehaving.
3 - Take the top cover off the backflow valve. On mine, the thing comes apart easily with just 3 screws and you can pull out part of the mechanism. Every time I've done that and put it back together, it's gone back to behaving again.
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I know it's been a few days so you may have already found out how to handle your problem, but if you haven't, try this forum:
http://forums.gardenweb.com/forums /
There are a number of lawn/garden related topics where you should be able to find someone who can help out.
Good luck.

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I moved the compressor very close to breaker and ran a short hose to the pressure port. This allowed me to blow out my sprinklers properly. The breaker stops "gurgling" after I give it enough airflow to "open it", at which point the air goes to the heads and they pop up. That point seems to be an indicated 65-70 PSI on my compressor output gauge. Once the heads pop I can immediately reduce the air pressure and the breaker will remain open with the heads remaining up. I reduced it to an indicated 50 PSI after the heads popped up. The heads will remain up even if I dial the pressure down to an indicated 25 PSI.
My compressor was not able to keep up with the airflow at 50 PSI so the pressure dropped slowly as I blew out the lines. I had to cut the airflow to allow the compressor to catch up a handful of times.
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