After going close through Charlie Savage during one sustained attack, a lovely, patient move yielded the opening goal in the 36th minute. Iestyn Hughes provided a telling right-wing cross and Daniel Gore powered in an impressive header.
The schoolboy's effort separated the two sides at the interval, with Salford still very much in the tie and asking questions of the Reds' defence.
Charlie McNeill added a brilliant second to seal United's progress. After passes by Savage, who had moved into midfield, and Alejandro Garnacho, the Under-18s' leading scorer turned sharply to get away from his marker and found the top corner from an acute angle with a confident finish.