what will cause an engine to shut down when brakes are applied?

When you slow down or come to a complete stop then the RPMs of the engine obviously go down but your RPMs may be running too low for the engine to stay running. Start the truck when it's parked and let it sit there running and watch the RPMs or listen to the engines RPMs if you don't have a tach. If it runs lower then normal when just parked or if you notice the RPMs flucuate up and down like the engine is reving by itself without touching the gas then that's the issue you need resolved.