DIY Overflow PVC Siphon

Hello!
My tank is coming along nicely I believe. Cycle is almost over!:whoopee!:
So, in the mean time, I have been exploring and reading about DIY projects for the aquarium. The first one I decided to tackle was a PVC DIY overflow project. I build three prototypes and finally, with the inspiration provided by many forums and websites, I designs a 1" overflow that works perfectly, at least I thought it did. :grumble:

The overflow I have builds runs silent, with no gurgling or sucking noise. I added 2" drain cover to the intake to keep my future fish out of it. I am going to add a screen to the back side of the drain cover to keep snails out, but that will come later. The siphon portion of the overflow moves water really good and steady, but is restricted by a ball valve i placed on the output line in order to control the volume. To start the siphon, I build into the pipe construction a standard airline from Walmart. This airline is used to remove the air bubble that forms in the upper part of the U when the siphon is broken. This worked perfectly, after three prototypes.:frustrat:

So, I ran the siphon on my tank for 6 hours and it worked GREAT! I thought I was victorious! Then midnight came along and I unplugged the pump from the sump to the main tank and let the siphon stop. Once it stopped I plugged the pump back in. It started again, as expected. I then unplugged it again and let it drain to the intake like before but this time I waited until about 7 in the morning to plug it back. The pump started but this time the siphon did not start! I primed it and away it went, working great. I did this over and over for a week.


When the power fails to the pump for a short period, the siphon is not breaking fast enough so it works fine. When it is off for a extended period of time, the siphon is loosing it's prime SOMEWHERE! The only suspect I can imagine is the airline.:dunno:

I then thought about it and decided I should plug the airline leading to the U after the siphon starts. I sucked the air out of the U until water started coming through the airline. I then plugged the airline to the hold the suction. It held for a entire night. When I started the pump again, it still had lost the siphon. Here is a diagram of the system I have build. Any ideas!!!!!? I'm about to go crazy!

What about adding a pump like this to the airline?
[ame=http://www.amazon.com/Aquarium-Lifter-3-5gph-inches-height/dp/B001BOD9JI/ref=cm_cr_pr_product_top]Amazon.com: TOM Aquarium Aqua Lifter Pump 3.5gph flow rate to 30 inches height: Pet Supplies[/ame]

I am not very familiar with this kind of overflow, but it seems that some of your pipe connections might have a small leak. Enough that it will break siphon after sitting for a while. I would spread some more primer and glue around all your joints to see if that will help.

UPDATE:
Last night I attempted something different. I followed the same procedure in sucking the water out to start the siphon and it did. This time though, when I start getting water from the air hose coming from the vent, I continued to suck water(with a syringe, not my mouth...lol). Each time I suck a little out, crimp the line, and suck a little more water. Each time I was getting small bubbles. I did this about 10 times until it was nothing but water coming out of the line. I then plug the syringe in the airline one last time and used it to plug the line for the test. It was holding a air tight grip of the water in the airline. I went to bed and 7 hours later, the siphon started again!

Though this of course not FAIL safe, it is a big leap forward. I am testing again today so when I get home from work, I will see if it starts up again. In addition to this, I am building a latching relay circuit that I will plug only my sump return pump into. This latching relay will have a manual reset button and will be held closed by the voltage from the outlet. If this voltage stops or drops enough, the pump will turn off until I restore power. The rest of the aquarium will function as normal. There is slim to none chance of a flood now Here's a picture of the PVC DIY Overflow I have build: