A hat-trick from Anthony Stokes and strikes from Russell Latapy and Marc Twaddle saw off a sad United.
United's misery was compounded when striker Noel Hunt was sent off for two bookable offences late on.
Brewster made five changes to his starting 11 and goalkeeper Derek Stillie was left out with Euan McLean taking over between the sticks.
Lee Wilkie played his first game for 19 months as the spare man at the back for United but it was a difficult return.
Stokes' gallop left David McCraken for dead and Latapy steered the cut back past McLean from only six yards.
It was just the start Brewster and United had dreaded. They did offer some response and Hunt felt he was fouled by Kenny Milne as he homed in on goal after David Goodwillie had sent him clear.
But it was Falkirk who always looked the more assured in possession. Patrick Cregg had two decent efforts from distance as they suggested they could add to their early lead.
The lively Hunt almost conjured up an equaliser 12 minutes from the break. Smith's shot struck him in front of goal but he had the presence of mind to dig the ball out from under his feet and direct a shot on target which Jeroen Lambers palmed onto his post.
There was no lack of fight about United and Smith epitomised the spirit as he chased every loose ball in midfield and made his presence felt.
But there was another calamity around the corner for United. David Proctor lost his footing in his own area and Stokes pounced to slip the ball past McLean from a narrow angle.
Stokes grabbed his second after 48 minutes after hesitation by Wilkie enabled him to score with a 16-yard shot.
Collin Samuel pulled a goal back after 69 minutes – four minutes after Hunt had been red carded – but Twaddle scored from close range five minutes later to make it 4-1.
Stokes then completed his hat-trick from the penalty spot after he had been felled by Wilkie.