when i go to start my nitro truck, when the engine turns on the wheels start running and when i hit the brakes the engine tunrs off? Is it my idle?

Sounds like your throttle/brake linkage is set up wrong.What kind of vehicle is it?

While the throttle/brake linkage may indeed be set up wrong that is not why the vehicle dies when applying the brake.
Your problem is most likely your idle, more speciffiaclly your idle gap.
When applying the brakes you fully close the carb. If you do not hav a 1mm - 2mm idle gap the engine will be starved for air and stall. The idle screw is usually located near the base of the carb.
Having said that, there are other things that can cause an engine to stall when applying the brakes. Things like broken clutch springs or damaged/sticking clutch shoes will do it too.

WHAT HE SAID
also check your low speed needle. if you can go back to factory settings and retune, get the car at operating temp. then set ur high speed needle first ,once high speed is set and NOT TO LEAN then adjust idle if neccesary, then low speed needle do this by pinching the fuel line , it should run for 2-3 seconds rev high then cut out , if not then richen a lil bit like an 1/8th turn run it high speed again and try the pinch test again.make sure u run the car each time, to clear any extra fuel that may load up and give you false settings
make sure the car is at operating temp throughout this procedure.
of course keep in mind it could be more than jus setting the enginie to run rite it could be jus a daveg55 said . check out your vehicle and start from there. if you dont know how to tell or what ur lookin at take it to a LHS (local hobby shop)
good luck
Q

Dont seem like he needs a hobbyshop to me.Seems like daveg55 has it all figured out[X(].How do you know thats not why the vehicle dies?It could be 10 different things and none of us have seen the vehicle,therefore all ANY of us can do is offer advice and suggestions.Dont come on here being so MATTER OF FACT,downplaying the next mans effort to help.You guys kill me playing the rc god[:@]

How do I know that's why????
Because if the carb were set up right the engine would not die no matter what the linkage does. How do I KNOW that? Nearly 40 years of playing with and working on nitro powered engines, that's how!!!
You are right about 1 thing, it could be ten different things And I pointed him in other directions too, directions far more likely to be the problem than the linkage.
Since you were so generous as to give me the advice of not being so matter-of-fact and playing rc god let me return the favor.
You should grow up and try to keep from getting your panties in a wad when others advice disagrees with yours.[:@]