It's the rubber gasket between the MC and the power booster. Actually slides in to the booster? Searched high and low...it is a 10" Delco Booster

HI. THAT IS A CUP SHAPED SEAL FOR THE BOOSTER. WHEN IT IS BAD, IT WILL AFFECT THE BOOSTER OUTPUT, AND MAKE THE CAR HARD TO START, OR EVEN BACKFIRE WITH THAT KIND OF VACUUM LEAK. WE HAVE IT. YOUR COST $10.00 PLUS SHIPPING

Thanks Bob...I have been looking everywhere for that part and know one knew where to get it...I'll call your office today.

Welcome to TC

Powerbrakebob The 64 el camino listed in your title, Was that factory installed ? COPO ? Or what??