For some reason the Metro trip planner site is not handling this route well.

It's Metrolink all the way, but will require a transfer at Union Station. Take the Metrolink Orange County Line north, all the way to Union Station, then transfer to the Antelope Valley Line toward Lancaster. From there, it's probably easiest to get off at the Sun Valley station, then walk out to San Fernando Road and take bus #94 up into Pacoima. It depends on where you're going in Pacoima though, but that's probably the quickest way. Should take 2 hours or so, depending on transfers.

Regular fare would be $11.50 for the one train ticket. You get free transfers to local buses with a Metrolink ticket, and #94 is a local bus.

According to Google Maps, you can also go to the Sylmar station and take bus #224 back to Van Nuys Blvd. That's a little out of the way, but may be faster.

Hope that helps.

Sorry, looks like the #94 transfer would only be on the weekends, while the #224 (from Sylmar) would be on the weekdays. Sorry if that's confusing. Should have trusted Google Maps all along.