one main drain bubbling more than other main drain when blowing out

[lobes]

1st time pool closing by myself and I am in the middle of blowing out my main drain with a pancake air compressor set at 25psi. I have 2 main drains and when i continuously blow out with compressor, one main drain bubbles continuously very strong and the other main drain intermittently bubbles. Regardless, while main drain bubbling I closed the main drain valve to air lock and immediately shut off the compressor. My main concern is if i properly blew the main drain with the air compressor.

I also tried to put antifreeze directly into the suction plumbing but I did not see the antifreeze come out of the main drain. I did see the antifreeze when i blew out the skimmers though.

 

[PoolguyinCT]

It’s normal for more bubbles in one drain.

 

[lobes]

Do you think I should be concerned I didn't see any antifreeze blowing out of main drain? I saw antifreeze blowing out both skimmers.

 

[PoolguyinCT]

No - & I wouldn’t use af in the drain- it’s heavier than water

 

[lobes]

Thank you for your prompt reply. Now time to blow out the returns.

 

[ndakotaninja]

I believe the 2 main drains are directly connected together by a pipe and the second drain is for safety. If something blocks the first drain, it can still pull water in through the second drain and doesn't form a vacuum, holding whatever (or whomever) is blocking the first drain. So when you push air back out, it comes out the first drain, and whatever additional air that can't get out the first flows to the second drain. You typically only get a small amount of air from the second drain. At least that's how my in-law's pool was built.