Allan, who spent last season on loan at Hertha BSC, played his first game of pre-season and enjoyed a 45-minute runout at the Kirkby Academy.
Brewster appeared as a second-half substitute as the 17-year-old made his comeback after suffering a broken bone in his foot during the summer.
With the new Premier League 2 season starting away at Swansea City next Friday night, this was a good opportunity for U23s boss Neil Critchley to give his charges more minutes as they stepped up their fitness.
Dutch winger Bobby Adekanye went close in the first half with a curled effort that came back off the post and Cameron Brannagan fired a shot straight at the goalkeeper from close range.
Adekanye had some lively moments on the right flank and Brewster dragged a decent opening wide of the target, but a late goal on the counter-attack saw the Welsh side claim the victory.
Reflecting on the defeat, Critchley told Liverpoolfc.com: "First of all, it was good to have Rhian available again. He only started training again a couple of days ago but it was important to give him some minutes along with Allan.
"We were disappointed with the scoreline. We dominated the game for large periods but got caught with a sucker punch right at the end.
"We need to be a bit sharper going forward because we didn't do enough in the final third."
Liverpool U23s: Grabara, Whelan, Hart (Juanma, 60), Phillips (Johnston, 75), Jones (Masterson, 75), Williams (Parker, 75), Adekanye, Virtue (Alves, 65), Dhanda (Brewster, 65), Allan (Kane, 45), Brannagan.