Could be a few things, like swarm said. First, check your idle adjustment. Remove the air filter and turn the truck on, but don't start the engine. Pull full brakes and look into the carb throat. There should be about a 1mm gap still visible in the throttle body. If there's no opening, then the problem is you're completely closing the carb under braking and cutting the engine. Adjust the idle stop screw until you have about a 1mm opening when at full brakes.
If you have a decent opening, then you may be running your low-end too lean. And when you hit the brakes, then engine gets starved for fuel and quits. Richen the LSN in 1/8 turn increments until more normal performance is seen...