Purging air takes so long...

Recently I added a few rads to my OFCH system and 2 rads don't get hot.

I have been letting air out of one of the cold rads, hooking up a plastic tube to the bleed nipple and placing the end in a bucket of water, so I can clearly see the air bubbles.
I have already drawn off a few buckets of water and the bubbles just continue the same.

I turned off most of the rads on that line and then the rad I'm bleeding gets hot, but the air bubbles persist.

I'm wondering if there is any point in continuing to bleed the air, since the bubbles just seem to continue...?

still purging the air, One of the pipes in the garage that feeds the 2 cooler rads goes down about a foot, then up again, which might make it more difficult to get the air out? Do you think I should change the pipework, and avoid the downward bend, and give a seperate feed for each rad from the main groundfloor zone pipe.
Or should I just continue bleeding a couple of hours a day? The rads are getting hotter, bu it's so time consuming all this bleeding..,

Are you bleeding into buckets? You need to bleed into a drain, a sump, or even your house trap. With a large loop you could be at it for hours trying to bleed it with buckets.

yea basically bleeding into buckets. I suppose it would be better to bleed into a drain as you say. The thing is that I have to keep increasing the water pressure in the boiler every 15 mins or so, which make it a bit inconvenient.

Today I was bleeding the coldest rad, doing fine ..getting quite hot, then when i stopped bleeding, it just went cool again, so now I think it will be faster to re-do the pipes in the garage so that I have seperate feed for the 2 rads and remove the downward bend in the pipe, making it horizontal more or less.

For some reason I didn't realize you were bleeding radiators, I thought you were purging baseboards. Usually a cup or so of water is all you need to get from a radiator before the air stops. If you are getting more than that you have a problem somewhere. Probably exactly where you think.

I have taken your advice and now I have the water flowing into a sink instead of a bucket. So I'll leave it draining very slowly for a day or so and see if the air bubbles disappear, before re-doing the pipework as a last resort.

The boiler is off, so I just let gravity from the attic tank supply the pressure, which is quite low at around 1 bar or less, but that's about all I get from the height of the attic tank.
I assume that if I get rid of the air, it wont sneak back in?

I stopped the purging and thought I will just re-do the pipes tomorrow, but then when I turned on the heating, the rad actually got quite hot, even though I still see some air bubbles when I connect the little tube, so I might just bleed a bit more , though the rad does have an automatic bleed nipple too which I suppose over time will get rid of all the air. Thanks the advice was appreciated.