Chances were difficult to come by for the visitors as Montreal's stubborn defence made life difficult, but despite being starved of service for long spells, Bradley Wright-Phillips threatened when he headed wide from a Sam cross.
Despite still not having new signing Didier Drogba available for selection, the hosts were growing in confidence at the other end of the pitch and only a fine save from Luis Robles kept them out as he reacted impressively to get a hand on Dominic Oduro's low strike.
Oduro would not be denied late in the first half as he exchanged clever passes with Dilly Duka before breaking the deadlock with a calm finish inside a congested box.
However, the Red Bulls were back on level terms six minutes after the break when Mike Grella's inch-perfect cross reached Sam, who managed to bundle the ball home for his sixth of the season.
Shaun Wright-Phillips was introduced from the New York bench midway through the second half, but opportunities were rare as the away side looked for inspiration from the brothers in the final third of the pitch.
The visitors were handed a boost late on when Montreal's Laurent Ciman was sent off for a second bookable offence, but they were unable to force the issue in the closing stages, which means that DC United still hold an eight-point lead at the top of the Eastern Conference.