From my experience, if the pipe is not at an angle, and I mean slight, then the water does not flow to the end very well. When I say angle, the end closest to the bucket should be a tad higher than the other end. If not, it can stop half way, or if you happen to put the end up higher than the end by the bucket, it will not flow past a certain point. If you put it level, then the nipple at the end may not get a good flow of water to it and you have the air bubble thing you spoke about. The reason I use the low pressure is becasue I do not have to worry about air in the lines and putting the pipe at a slight angle. Also, unless you put a water valve in the buck for gravity feed you have to fill up the bucket by hand.
Making a low pressure valve and running it off direct water pressure was easier for me.