It's tough to find dirt roads in the wild in the Bay Area. Too many gated hippie hiking trails.
The loop I like is in the southern Santa Cruz Mountains. From Pacifica:
* Head south on Hwy 101 or 280. Basically, just get to the Almaden Expressway exit on Hwy 85 in San Jose
* Make the first left at the exit and then an immediate right (onto Almaden). Keep going straight (~4 miles) until it ends on Harry Road.
* Turn right on Harry and make an immediate left onto McKean
* McKean becomes Uvas
* Enjoy the views and sweepers along Uvas
* When Uvas ends, turn right onto Watsonville Road
* On the right shortly is Chitactac-Adams Heritage County Park. This is a great little park to visit, btw. Restrooms and a nice little interpretive trail.
* Just past the park, turn right onto Redwood Retreat
* The road forks; turn left onto Mt Madonna
* FINALLY, the pavement ends! Enjoy 2 miles of dirt/tree roots/mud/fun through the redwoods.
* At the top of the hill, turn right onto Summit Road
* Keep going straight even past the signs saying the road ends.
* The road turns unpaved again for 10 glorious miles.
* You'll eventually hit an intersection at the top of the hill. Turn left (Loma Prieta Ave)
* Turn right onto Loma Prieta where you see a cluster of mailboxes on the right (I almost always miss this turn; GPS is your friend). More unpaved goodness.
* Loma Prieta eventually intersects with Summit Road near Hwy 17.
* If you miss the Loma Prieta turn, you'll stay straight on Mt Bache, which intersects with Highland Way, which becomes Summit Road. So no worries unless you really want that 1 more mile of dirt. ;)
Anyway, it's about 90 miles round trip (15 miles of dirt).