I have a nice 5 year old, been clocking in the 2d, I have noticed that when I have to pull on him a little more he does not like the pressure from the curb, I have ran 3 nice runs, added some speed and he needed a little help around the barrel and I had to help guide him around and he did not like that pressure under his chin, so he kinda braced and tried to just run, but I did get him calmed and he came right back down.

What do ya'll use in a situation like this, thinking of trying a leather curb just hoping I still have brakes.

I use a thin twisted nylon rope/cord on some of my bits, I dont know the thickness of it.

You could supplement a different curb like a leather one but if that doesn't work you could work on changing his reaction.  I'm a firm believer that if they don't like the pressure when you up the work load, in this case adding speed, there is a hole somewhere. I would practice getting a hold of him in practice runs and getting him to tolerate you being to manipulate him during the run.   In my opinion, if you can't help a horse when you have to during a run, it's a huge disadvantage because when something goes south, winning and losing maybe the difference between if you can help him or not during a particular part in your run. 

Southtexasponygirl you still have enough whoa?

Wetsaddleblankets, I am working on it in the practice pen,  maybe after some time we can get past it, kinda thought about using one of those gel curb  covers just to ease the bite a little till he can get over this, if he does

On one gelding I did'nt have much whoa he had a harder mouth and I bought him that way so had to use a stronger curb on him, on the other two that I used the twisted string/rope had really soft mouths so did not have a problem with stopping. 

I ride my sensitive gelding in a chain mouth Rosie gag with no curb.  He is off the track, but will stop when you set down and say whoa.

I really like leather curbs!

If this were my horse, I would not be adding speed if he can't handle pressure from the bit. JMO. I agree with wetsaddleblankets that the horse needs to accept bit pressure and be soft to it. It's basic training fundamentals.

I know everyone has their different training methods but I always feel like if my horse takes a split second to lean on me, or pull on the bit, or brace, or anything like that -- now I've just lost the 1/2 second that could have been at the top of the barrel race. There is no "spare time" in the run for things like that to happen.

This is why, for myself, I do lots of other things with my horses (English, reining, ranch horse, etc) so they learn to do other things in the arena and always learn to be soft and accepting with that bit (and the rest of their body) all the time. I know that's not everyone's cup of tea, but it works for me.

So I again would recommend to take a step back in her training, back off on the speed/compeition right now, and focus on the training problem instead. Fix that, and then you no longer have a bit issue in the long run.