Leroy Lita kept City in the FA Cup with his 16th and most bizarre strike of the season just seconds before the final whistle.
Brentford had more passion and after taking an early lead dominated most of the tie until City's Steve Phillips punted the ball deep into their area.
Stuart Nelson came off his line, slipped, and left Lita to tap in a most unlikely equaliser.
Veteran John Salako had given his side a perfect start with a free-kick from just outside the area after seven minutes.
Isaiah Rankin had been bundled over by Tony Butler as he rounded the centre-back and the defensive wall was rooted as Salako's left-footed shot was sent with lethal accuracy over their heads.
City had an almost identical free-kick minutes later but when Mickey Bell went to hammer it through the wall, Kevin O'Connor deflected it firmly out for a corner.
A lively Brentford side were soon back in control putting Butler and his partner Matthew Hill under constant pressure.
City escaped when Butler was again penalised outside the box and were then kept in the tie by a marvellous save by Phillips from Rankin after Deon Burton had nodded across goal.
Alarm bells were ringing for City as player-manager Brian Tinnion went on a few warming-up runs.
City held out until the break when he replaced Bradley Orr in midfield while Butler was also substituted in a reshuffled defence.
Both goalkeepers found themselves on yellow cards, Phillips in the first half for dissent and Nelson in the second for delaying a free-kick.
Chances for the home side were rare, Steve Brooker getting in their only shot in the first hour.
Brentford manager Martin Allen almost joined his goalkeeper in referee Andrew Hall's notebook as he jumped up to the touchline to express his frustration as the second goal refused to come.
Rankin went close to giving him it while Lita headed against the bar as City threw everything into a wave of desperate late attacks.