Jake Kelly's first-half goal was the difference at the Carlisle Grounds where the League of Ireland side saw off the Red Devils Under-21 squad
Warren Joyce's young Red Devils included winger Bebe and senior squad member Tom Lawrence, but fell behind on 42 minutes when Ismahil Akinade swept the ball in for Kelly, who struck the ball past Joel Castro Pereira and into the net.
Bebe had a decent chance for the visitors to level the game before half-time, but was denied by Bray goalkeeper Stephen McGuinness. United changed 10 players at the interval, with Lawrence working hard from the bench.
The 21-year-old forward forced a good save from Wanderers substitute Shane Redmond, while Davide Petrucci and Saidy Janko also kept the goalkeeper busy.
However, the Seagulls held on for a good win, which they will hope to use to build confidence ahead of the resumption of their SSE Airtricity Premier Division campaign.