I made the mistake of trying to clean out my old needle and seat on my 600 Holley even tried a couple from my other carbs, finally took the advice of my son and put a new needle and seat in, fixed it 1st time.
An old timer who founded Product Engineering which several name brands adopted the design features of his pumps almost t the "T' told me only about 6 out of 10 new needle and seats can hold back up to 11 psi max.