Kevin Finlayson was short with a headed pass back and keeper David McGurn had to charge out to retrieve the situation, colliding with and injuring the on-rushing Iain Russell who was unable to continue.
Replacement Paul McQuilken settled quickly and fired a rising shot wide of McGurn's right-hand post.
In the 20th minute Chris Millar sent a low free-kick to Derek Lilley at the edge of the box. The striker's attempted drive was blocked but the ball fell to Jim McAlistair who thumped it past Stephen Grindlay from 20 yards.
Dumbarton replied spiritedly and McGurn had to make low saves from Ryan Borris and Andy Rodgers.
He had to make an even better one, diverting a powerful low shot from the dangerous Rodgers after the striker had sprinted clear of the home rearguard.
Peter Weatherson scored a clever second eight minutes after the break. He cut inside Craig Winter and Chris Boyle before slamming a low left-footed shot past Grindlay from just inside the box.
McQuilken and Winter came close as the Sons refused to buckle, but two goals in a minute from Jason Walker sealed their fate.
First he turned to fire an unstoppable shot high into the net after Weatherson nodded on a Scott McLaughlin corner and then he swept home a James McPake pass from 10 yards.
McGurn denied Boyle a late consolation with a fine save from the midfielder's searing drive.