The Timbers' Steven Smith was called for a handball inside his own penalty area, setting up a chance for Corradi, who left Portland's Joe Bendik with no chance to stop a shot fired high and down the middle.
Substitute Sinisa Ubiparipovic wrapped up the win with an insurance tally in the 84th as Montreal (2-5-2) earned its seventh point in four matches.
Portland (2-5-1) lost starting goalkeeper Troy Perkins midway through the second half when he took a blow to the head.
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