I think you need to figure out why the air bubble is there and how to stop it from comming back again. A correct siphon should not have an airbubble in the line, so something is not right, but I am sure you realize that. If all your joints are sealed, then perhaps the water is not keeping the pipe full enough, flowing fast enough, from the end and letting air seep back in? Maybe you just did not get all the air out to begin with and it slowly craweled up hill to the highest point, fighting the water flow of the other direction. Sometimes that happens, many smaller bubbles in a line gather at the highest point. I never did a big siphon like yours but in moving liquids I use normal siphons and hoses alot. 15' drop for 160' of pipe does not seem like alot, depending how it lays, if the end is level or has the drop etc.. I would think perhaps the air is entering from the outlet. If that is the case then maybe making the end just a bit smaller would stop the problem, maybe cap the 6" pipe to only 5.5" opening. How did you measure and where, the PSI as 6.5?
Any air compressor can basically be used as a vacum pump also, just connect to the air inlet and you have suction. How to do this varies with compressor design. I think you could mod one of those little 12V car tire compressors that plug into a cigerrette lighter easy enough. Your not wanting to draw a large vacum, just remove standing air so it should work good for that. The thing about compressors used this way is the piston is made to work hard compressing air, not sucking air, so if you were trying to draw a heavy vacume you could shatter the piston, even in large heavy duty compressors same thing. Just to remove air though it should work fine.
If you have a junk fridge or freezer handy and power available many people use the compressors from those for vacum pump also.
Got a shop VAC? Preferrably a wet dry type
As for a hand pump? Whats that
I have not used one of those for years for anything but ballons when making twisty animals. If you have one or can find a cheap one to buy, find where it sucks in the air, you should be able to install a fitting in the hole and attach a hose. I remember one I had years ago that had a small hole in the top cap where it sucked in the air, close to the shaft.
Also I have no idea how much air you need to remove or how you will connect the vacum pump etc.., but perhaps one of those small airpumps for the ballons would also work?
Wall-mart (and other places) were selling about a dozen twisty balloons and a small air pump for $2. Get rid of the air bubble and get a new hobby at the same time
For $2 the ballons are ok, the pump was decent, and it comes with a small book how to twist several animals like a poodle. I don't have one handy to look at, but I think the pump could easily been modded to suck a small vacum, just plastic so it's easy to put a hole anywhere and connect a hose. All my Clown and Magic stuff is put away or I would take a lok at one. I know I used my professional ballon pump for a vacum a couple times, the $2 ones are just a much smaller cheaper version really so should also work.
I was giving away those $2 sets, I think they were in the toy section of the store, not the party section. I used to buy them by the dozens. Probably the only thing I could never find a wholesaler for