According to Pete Battistini's "AT40: The 70s" book, the show began on KNEW in 1971 and ran through '72. Then KYA picked it up in '75.
The book also says KSJO in San Jose had the show in 1974-75.
I moved to the Bay Area in '79 and clearly recall hearing AT40 on KYA (AM and FM) until 1981. From there, it moved around a bit:
1981 - early 1982: KCBS-FM (prior to it becoming KRQR)
1982 - 1984: K101
1984 - 1987: KITS
Not sure of anything post-'87.
I recall KITS running the show even though it had evolved into a full-blown alternative station. I think they buried the show at 6am on Sunday; perhaps just to run out a contract and fulfill the spot obligations?