Until the tunnel under Mulholland Highway opened in 1930, there wasn't much "pass" to Sepulveda Pass. There was a dirt road and trails, but most of the traffic between the Valley and the city moved over Cahuenga Pass and narrow passages like Laurel Canyon (and I think Beverly Glen.) This photo of the tunnel's dedication is from the family archives of former Hollywood stunt double Sylvia Durando, whose mother is on one of the horses. I posted the pic at The Valley Observed in 2006:
Descendants of the Sepulveda rancho family attended the ceremony and the San Fernando High School band played. The program contained ads for Valley chicken farms and the brand new Los Angeles Metropolitan Airport, "at the Heart of the San Fernando Valley, Woodley Street and Saticoy Boulevard." That's now Van Nuys Airport.
Sepulveda Pass was paved and became a state highway route in 1935.