Is this possible? Pure breed from mixed flock.

Depends on your roosters, no guarantee the ones hatched will be purebred since phenotype can hide the true genotype. If you only use leg horn roosters and hatch only white eggs then you will keep the breed pure. This is what I do with my EE, only EE rooster hatch only green eggs.

 

It depends on how accurately you can identify the the mixes. If you're 100% accurate at eliminating the mixes, then 100% of what remains will be pure. If you miss some of the mixes and then include those in the next generation of breeding, the purity of your purebreds will spiral downhill.

 

I guess the biggest thing to remember in the the chicken world we prefer to have standard breed chickens vs "purebred" chickens. We can have some mixing in our breeds but remember we should be breeding to the standard and more breed crossing you do the further away from the standard you get.

So if what you are doing is trying to keep a line of Brown leghorns that are breed as close to the standard as you can, by selecting those that meet the standard you should be ok.