Crop bra won't do anything for sour crop. Sour crop is just a bad yeast infection occurring in the crop, and yes, dark brownish liquid spilling out of a very full crop is definitely a sign. Crop bras are meant for chickens with pendulous crop who need help holding the crop up, this does nothing for a fungal infection in the crop obviously.
Chickens who do not have sour crop but the crop is too full will just spit up undigested feed/water (you can tell it's feed.) It won't look like dark brown liquid like you're describing.
Go out and buy monistat 2% from the grocery store or something, off-brand is fine (and cheaper/same thing.) Once you have it, try to drain your hens crop as much as you can to get out as much as that yeast as possible before the first treatment. Rub her crop, when she starts to gurgle up liquid, tip her upside down [forward, like you want to puke her] briefly and push the crop and try to expel as much as you can as fast as you can (you don't want her to choke, that will kill her.) Do that for less than 5 seconds and then let her catch her breath - it helps to remove liquid from their beak/have a towel ready to wipe the beak, the hole they use to breathe/fill their air sacs is located right next to the hole they expel liquid from so get the whole beak clear so they can breathe again.
It doesn't need to be fully drained, just try to get a lot of the liquid yeast out at first so the medicine can hit the walls of her crop initially and not just float around in a pile of liquid. You should only need to tip them like one time to get rid of the initial yeast blob, you don't want to do it too much because it's a risky move that can choke/suffocate your chicken.
Then give her about one inch of monistat, my chicken will just eat it so you may not need to force feed it. I just offer it to them like it's a treat and they eat it. If she's not interested, I just mix it with a little yogurt and they'll eat that.
Then massage the crop to try to spread that monistat around all over in there. Give her monistat in the morning, and again at night (twice daily) for 7 days. Make sure you do it for the full week even if it seems like it's gone earlier, yeast and other fungi are very resilient to treatment and will grow rapidly if not fully eradicated from the problem area. You don't want to go through treatment all over again haha!
Sour crop is often caused by other issues, such as blockages or worms, so make sure there isn't anything like that going on too because it will reoccur if you do not get rid of the blockage. A blockage will cause tons of food/water to sit in the crop, which is a perfect environment for yeast to grow [this is how sour crop occurs, and why it's often linked to blockages.] Your massages may get rid of the blockage though during the sour crop treatment, hopefully.
