a trick for finding vacuum leaks is to start the engine and carefully spray a flammable liquid (one that evaporates quickly, like starter fluid) along the lines and seams. an unlit propane torch works too. if you find the vacuum leak, the stuff will get sucked in and the idle speed will change.
obviously have the means to put out a fire close at hand :yes: , but I've not yet heard of anyone catching their truck on fire using this technique.