The secrets to successful braintanning of bear hides are to get help unless the bear are small, & work fast & keep the hide cool.. You're in for a lot of work.
I salt every hide before fleshing, & that means literally cover the hide with salt at least 3/8" deep on the 'meat' side of the hide. Keep the hide cool, ( I have a fridge just for this) let the salt sit for at least 8 hours, then get the salt off & flesh - work fast on this part. You will need to wrestle the hide, full of salt & rolled into a bundle, into the fridge unless you're doing this during cool weather.. Or have a cool place to work. That means in the 40F's. The salting helps 'set' the hair so it tends not to slip, but it's not entirely foolproof - nothing is. Best time to work on these very thick hides with thick hair is when the weather is cool, because of their thickness, so once they warm, they tend to stay warm longer than you'll want.
My stretching frame is made of 4X4's & I use heavy black automotive bungee cords to help keep the stretching lines taut when I'm breaking the hide. You'll know when the hide has broken uniformly, because the whole meat side will have become a nice white shade. Help is good here, too, because you can take turns, take breaks, & you don't get overwhelmed with the project.
When you apply the warm brain mix, you want to keep it at 100 - 105F. No hotter or it will not work right; no cooler or it won't go into the hide enough. And work it, work it, work it in. I use the paddles for that, I use for breaking, for this, too. I use a potato masher to mash & mix the brains. Have fun..