Check valve in brake line. What a DUMB idea!

Upon further information, turns out that little ball is not a check valve, but merely a way to hold the nut on. In any case, avoid these types of lines at all cost.

thanks for the update.
my google search could not find a residual check that look any thing like that fitting

What exactly was the problem, this is a relatively common swivel application in AN fittings.

Perhaps a cheap Chinese knock off?

The problem is sealing them off. If you don't do it properly you can go mad trying to get air out. They won't necessarily leak, but won't seal either. Much easier to use lines that don't use that little ball. And yes, they could just be a knock off.

Using a circular pin to hold the nut in place is common practice in AN hose ends (fittings). It acts like an internal snap ring and has nothing to do with the seal.

Typical AN hose ends and AN fitting adapters are 37° flare and should not be mixed with industrial 37° flare fittings or anything else. AN 37° flare and industrial 37° flare fittings function identically. In many cases they appear to be functionally interchangeable, but they are not.

The sealing happens between the male 37° flare of the AN adapter fitting and the female flare inside of the AN hose end. These are carefully engineered to work together.

If your assemblies are leaking it can be caused by several things like mismatched fittings (don't assume that a vendor is selling compatible parts), dirt between the sealing surfaces (doesn't take much) or the assemblies have been over tightened.

It's prudent to buy all parts for the assembly made by the same manufacture like Earl's (now Holley I believe), Fragola Performance Systems or any other reputable manufacturer. Then you don't have to worry about compatibility. You can mix an match between brands but do your homework. Don't assume that a vendor selling a car specific kit has it right. Ask what brand the parts are in the kit. If they don't know or won't say, go elsewhere. Plumbing issues can be deadly (brakes) or dangerous (fuel).