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Air getting into the brake lines, but from where?

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After replacing both front flexible brake lines and tightening everything back up I go to start flushing all 4 lines as the fluid is due to be flushed and there are tiny bubbles galore vacuum bleeding at all four brakes. Because I changed out the front lines I bleed those first just to get fluid in there. Although I keep the fluid reservoir filled during this whole process I get lots of continuous bubbles starting with bleeding the front lines, and then with both the rear. Working the hand vacuum pump through all this got Really tiring as 95% of what was coming through was bubbles. But I was determined to get all the old brake fluid out so I end up running just short of a 32oz can through all the lines until the fluid is clean, but I still have the bubble issue.

Only thing I can think of is that I got turned around somehow and when I went to loosen the first old front brake line from the solid line I ended up tightening the compression fitting (?) about 2 full turns instead of loosening. Not having replaced a brake line in a while I'd forgotten brake lines generally loosen right up once they're cracked. But dumb me forced that D#!& nut around twice and I'm guessing ruined the threads at that connection, and now that's what's giving me all the bubbles. The hex part or 'nut' I over-tightened is on the hard line that comes in from the left in the pic below.

Only 3 places I'm sure the air ISN"T coming from is the reservoir (as it was always filled), the soft line connection point top bleed screws, or where the vac line attaches to the bleeder screws as I watched the air come directly out of all the screw holes.

Note, the picture shows some brake fluid on the threads of the nut I over-tightened. As a test I wiped the fluid off then pumped and mashed on the brake pedal hard (spongy) but no fluid came out again at that connection, it stayed dry. I guess it was on there from when I put it back together.

Any advice on what to do next or how to figure out if the bubbles are coming from the connection I may have ruined?

'02 Taurus with 100k miles btw. Thanks.



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I would empty the system, then pressurize said system with air and check for leaks, using soapy water. Best Wishes!

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I would suspect the master cylinder or booster.  See if the pedal is firm.   Also pump the brakes with the system full - if air is getting in fluid will get out.

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