I have a 1972 c 10. 350 5.,7 4 barrel was
wondering if my rubber gourmet was
cracked where pub valve goes can cause
my truck to run terrible
Cracked or broken rubber lines usually only affect the vehicle around idle.....the performance at speed might be affected some if it is really bad. Owners have various ways of finding vacumm leaks......spray WD-40 on joints, connections and other suspicious areas. if the engine speed changes when you spray an area, take a closer look. You can also use starter fluid, even water.
What is a pub valve?
If you are talking about the PCV then I would replace the grommet but it would not be the source of your vacuum leak.