Air in Oil Feed Line to Furnace

Furnace technician spent 5 hours trying to fix this problem (to no avail):
At start of season, I purge my single-line oil system by turning on pump and open the bleed value, to fill the line with oil and get rid of any air.
I have a tank about 4 ft below the pump at the furnace, with the single copper supply line going across the top of the basement, about 6 feet above the pump. A standard filter is in the line 3 ft from the pump. The filter was changed, along with the rubber lid gasket.
But somehow, air gets in the line. After purging, it's OK and starts nicely and always and continues to run. But when left overnight, air enters the line and the shutoff kicks in.
When I then open the air bleed to purge the line (with the pump turned back on), foam comes out for a lengthy period of time.
I'm presuming that during the night oil is draining back down the supply line into the tank, which is below the level of the pump, filling the 20 ft supply line with air.
Worse, when I try to bleed or purge the air out of the pump and line, what comes out is intermittently foam, then some fuel oil with a bit of air, and occassionally straight, clear tawny fuel. I can bleed out a half gallon of fuel, but the problem remains.
Can the problem be solved by putting some sort of check valve on the supply line near the pump, to stop the back flow of oil back into the lower tank? Where do I get one of these check valves? What do I ask for?
And of course, from reading the literature on this, many are going to say that I need to simply create a two-line system, to send the excess air back into the tank.
But since the system runs well when there is no air in the feed line (after extensive purging), I'd like to at least try a supposedly cheaper check-valve in the line first..
The tech guy said that this was the weirdest, most difficult and enigmatic problem he's ever encountered.
What are the (probably many) soutions to be tried? I'm at whit's end.
--John
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I hope he wasn't charging by the hour.

ummm...yeah, ok

ok... so the problem is pretty obvious.

Just verifies the problem

ummm....no, only 17 feet of the line

Because your screwing with symptoms and not the cause.

ummm....no
ummm....no
You can *do* anything you want... throwing parts at it isn't gonna fix the problem.

Stormy?? is that you??

What about the other half(wit)
Here's a clue...... assuming the tank is full.....the problem is between the pump and the tank.
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Noon-Air wrote:

The tank is not vented properly.
You need a vent line that allows air to enter the tank as oil is being sucked from it. Without venting, a vacuum will develop that will tend to draw oil back into the tank when the pump isin't running.
The vent line must rise to an altitude higher than the pump output.
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Where did you get so damn stupid, hvacGayGuy?
Everybody knows the guy needs some new electrodes and a thermostat. Bubba
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I don't do much with oil, but this sounds like a really obvious case of either needing a new copper line from the tank, or a two pipe system.
--

Christopher A. Young;
.
.

"Noon-Air" < snipped-for-privacy@comcast.net> wrote in message
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On Sun, 16 Dec 2007 09:22:06 -0500, "Stormin Mormon"

Actually, Stormy doesnt do much with anything........thus, the bullshit answer he came up with. Bubba
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you say you changed the gasket on the filter...
is the filter above or below the tank?
is it possible the new gasket leaks air when the system is off?
Mark
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