Despite being boosted by a number of first team players who had played only a bit part in FRiday's evening's win at Glenavon, it wasn't a vintage performance by Gareth Montgomery's side.
United fell behind in the 51st minute, and could easily have been more than that had it not been for the superb goalkeeping exploits of Marc Maybin who produced a sucession of outstanding stops.
With 15 minutes remaining, in-form striker James McLaughlin struck the home side's equaliser when he confidentally knocked home a free-kick into the bottom corner from 25 yards for his sixth goal of the season for the Reserves.
This forced the game into extra time with little between the two sides, but with time running out it was McLaughlin again who popped up with the winner, heading home to send United into the third round.
The Sky Blues will face Derriaghy of the Amateur League 1B in the third round, which is due to be played on 11th September.
Reserves: Marc Maybin, David Reid, Lee Colligan (sub Ross Nelson), Gary Haveron, Ciaran McKenna, George Young, Lee McCaughren, Matthew Boyd, James McLaughlin, Michael Smith II (sub Paul McNeill), Aaron Lemon. Subs (not used): Aaron McGreggor, Neil Lowry, David McWilliams.
The Reserves are at home to Crusaders Reserves tonight (Tuesday, 7.45pm).