A memorable night at the home of the early season leaders propelled City out of the Sky Bet League One drop zone.
The Robins, rank outsiders at the start of play, capitalised on a slow start from hosts Leyton Orient to seal just their second away victory of the campaign.
Sam Baldock and Tyrone Barnett were both on target inside the first 11 minutes to leave the third-placed O’s with a mountain to climb.
It might have been more, too, before Kevin Lisbie pounced shortly before the interval to set up a nervy second half.
City rode their luck at times to preserve the lead, but ultimately ran out comfortable victors after Aden Flint headed the killer goal 15 minutes from the end.
Steve Cotterill made four changes to his side who drew 1-1 at Oldham on Saturday, with Karleigh Osborne returning from a hamstring problem to replace Brendan Moloney at right-back.
Ahead of him, Baldock occupied an unusual right-wing berth with Greg Cunningham on the left and Wade Elliott replacing injured namesake Marvin in the middle.
The final alteration saw Jay Emmanuel-Thomas restored to the starting line-up at the expense of Wes Burns, with City’s enigmatic forward justifying the back of his shirt in the ‘number ten’ role behind Barnett.
It may have been an unfamiliar approach, but the players responded with an immediate opener – the first time the Robins had scored the first goal in any game since the turn of the year.
Picking the ball up in the pocket of space between Orient’s midfield and back four, Emmanuel-Thomas turned and sprayed a wonderful pass out to the right flank where Baldock was waiting.
Although not a winger by trade, Baldock’s instincts were that of a seasoned veteran in that position. Running at left-back Gary Sawyer, City’s top scorer checked inside and dispatched a low left-footed shot inside Shwan Jalal’s near post for his 16th goal of the campaign.
And the Orient keeper was picking the ball out of his own net again inside the first 11 minutes, as Marlon Pack’s inswinging corner was powered home by Barnett just inside the far post.
It was a dream start for the visitors, and Baldock was intent on adding a third when set free by another diagonal pass, this time from Cunningham, only for Sawyer to this time read his intentions to cut inside.
Then Emmanuel-Thomas’ resulting corner tempted Jalal to vacate his line only to be beaten to the punch by Flint, whose glancing header just lacked the necessary purchase to hit the target.
It was all one-way traffic. The early season pacesetters were thanking referee Carl Berry when Derrick Williams’ deep cross was fumbled by Jalal and rifled home by Baldock, only for the keeper to be bailed out by a sympathetic decision.
All of the pre-match confidence had visibly transferred from the red-shirted hosts to their black-shirted visitors, who threatened to score again at the end of a slick team move involving Elliott, Barnett and Baldock, whose first-time pass looked perfect for Cunningham in the centre of the penalty area but the Irishman missed his kick completely.
Sooner or later City were going to need to do some defending. The home team’s first shot in anger did not arrive until the 39th minute, when Simon Moore comfortably smothered Sawyer’s effort, but triggered a much stronger end to the first half from Russell Slade’s team.
Midfielder Dean Cox was just off target with a 20-yard daisy-cutter before he caught one flush from even further out which forced Moore into an uncomfortable save and allowed Lisbie to steal in front of Adam El-Abd to halve the deficit a minute before the break.
Inevitably there was to be a very different complexion about the second period. Williams and Osborne both found themselves in the book – the latter for a crude lunge on Marvin Bartley.
But City were doing an admirable job of keeping Orient at arm’s length, with a blocked shot from Cox the only concerning moment of a dogged third quarter.
Admittedly a high proportion of the contest was now being played at Moore’s end of the ground, but Baldock was presented with a chance to ease the tension when Mathieu Baudry rolled a nonsensical pass across his own backline straight to the striker, who could only blaze high over the bar from 20 yards when outnumbered by retreating defenders.
Then City somehow survived an almighty scramble when Moore spilled Cox’s left-wing cross and Lisbie hammered the loose ball goalwards but straight into a posse of bodies on the goal line.
It was real backs against the wall stuff, but a moment of salvation arrived 15 minutes from time in the form of Flint’s second goal in two games.
The defender, sent forward for a prior set-piece, stayed in position on the edge of the box and capitalised on more suspect goalkeeping from Jalal, who misread the flight of Cunningham’s cross as Flint stole in to head City’s third.
Cox threatened to set up an anxious finish when he stepped inside and eyed up the top corner, but his radar was awry and Cotterill’s men were otherwise untroubled en route to a welcome away win.