Problem blowing out sprinklers

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:

Hi, I am not an expert at this, but you haven't gotten any other replies in the last 48 hours so i thought i would share my experiences. I have a 6 zone system that covers just under a third of an acre (sorry no measurements handy). I also have a Sears compressor - mine is 25 gal, 3.5 max/1.6 operating HP and a pressure up to 175 PSI. According to the specs mine is rated at 5.6@40 and 5.1@90 - so yours is pushing a great deal more air than mine is. I do essentially the same thing in that I pressure up the tank and run 50ft of rubber air hose to the point where my pressurized irrigation feeds into the sprinkler valve box. I have a special fitting I built from parts that connects the air hose to a hose bib on the irrigation system. I turn on one zone and plug in the air hose quick connect to let it rip. My compressor has no trouble in raising the heads on my sprinklers. I don't recall what specific pressure I set it to though. I do recall starting out sort of gently and then slowly increasing the PSI until it would lift the sprinkler heads though - i was afraid of blowing things apart initially. I guess i don't get the purpose of the 50ft copper tubing - is this a water trap or other? It seems based on my experience that you have the horsepower and there may be something else going wrong. Good Luck, Matt
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wrote:

It sounds like the air is going back thru and being released by the vacuum breaker or by the drain valve. When I blow out my system (considerably larger) I make sure to turn off a valve at the output of my backflow prevention device (BFD) so that the air has no choice but to go thru the valves and heads. This also eliminates any chance that the high back pressure from the compressor might damage the BFD. I also keep closed the drain valve until after I've blown out the system.
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The 50-ft copper coil allows the compressed air to cool before it goes to my water trap.
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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