The three nipple breather was, in1970, standard on California vehicles only, and, in 1971, standard on all 50 states application. The large hose connection to the air cleaner was to provide an air intake source for PCV system. The small hose connection was FROM the fuel vapor seperator located near the fuel tank and the middle or medium sized nipple was connected TO the carb's fuel bowl vent. The whole system used the ENGINE'S CRANKCASE as a storage place for all fuel vapors, to be purged, when the engine was started, thru the PCV system. During WOT conditions, when manifold vacuum is zero, the engine's blow by and any fuel vapors were directed to the air cleaner via the large hose connected filter housing. Later, this system was abandoned in favor of a charcoal canister with on demand purge, similar to the system GM used....far more effective and eliminated the possibility of a crankcase explosion. All this is explained in the 1971 FSM for the intended vehicle. Just my opinion of course.
BOB RENTON
