Moulton Harrox raced into a four-goal lead – but were left hanging on to book their place in the Lincolnshire FA Junior Cup final.
They looked in control as Joe Townsend (2), Ian Bradbury and Ashley Coddington left Brumby on the brink of a big defeat at Broad Lane.
But two defensive errors offered hope of a comeback and Brumby made it 4-3 with just over five minutes to go.
The visitors had another effort disallowed for a foul on the goalkeeper before Harrox could celebrate.
Assistant-manager Paul Crowson said: “We got three goals in 15 minutes before half-time which knocked the wind out of their sails after putting us under pressure in the early stages.
“They came out for the second half with nothing to lose but we got a fourth goal and then sat back.
“We still should have got more goals but our errors let them back into it.
“Overall, though, we are pleased with the result. This is our second final in three seasons and it will be my sixth county final with Mouton.
“We are on an unbeaten run and our target before the cup final is a top-three finish in the league. We want to carry on and there will be no room for complacency.”
Moulton have also won January’s awards for sportsmanship and team performances in the Peterborough and District League Premier Division.
In the other semi-final Swineshead Institute crashed out at Hykeham Town with a 2-0 defeat.
The Boston and District League Premier Division leaders struggled to adapt to the tight pitch and the home side’s long-ball game.
Swineshead’s usual passing game never materialised and they got drawn into long-ball tactics which didn’t suit them as they were deservedly beaten.