Inst ruined Bruff's unbeaten run last April when they won the play-off final at Donnybrook 32-23, but the Belfast men were second best this time around.
In Peter Malone's first game as player-coach, Bruff faced into the wind in the opening half and they were first off the mark when David Maw landed a penalty.
The Clem Boyd-coached hosts replied with a penalty from out-half Johnny Watt, who shared the next four successful kicks with Maw to leave the sides level at 9-9.
Bruff back rower John Shine then burst through for the game's opening try, getting in on the end of a pushover drive.
Maw missed the conversion and Inst had a short spell of dominance before half-time, with Watt mopping up with a brace of penalties to cut the gap to 14-12.
Both sides lost a player to the sin-bin as the half came to a close - Inst hooker and captain Wilbur Leacock saw yellow for persistent infringing and Bruff's Alfie Laffan followed him for a striking offence.
Malone's men started the second half brightly and five minutes in, winger Paul O'Brien sliced through for his side's second try after taking a looping pass from Maw.
The latter converted for a 21-12 advantage and with Bruff dominating on set piece ball and in open play and Inst having a second player binned, try number three soon followed for the visitors.
Maw was involved again as his reverse pass put Brendan Deady into space and he was able to barge past two tacklers and make the line for a seven-pointer.
The bonus point followed, with little over eight minutes remaining. Maw converted after his hard-working back-line colleague Brian Cahill had crossed close to the posts.
All that means Bruff top the Division Two table for the first time in their history.
Just a point behind them are a dozen teams - Dublin University, Highfield, Malone, Ballynahinch, Belfast Harlequins and Wanderers.
Dublin University, recovering from their recent AIB Cup exit, had the edge on Clonakilty in a lunch-time kick-off at College Park.
Michael Boland kicked two penalties and converted Brian Coyle's try for the students' third win in four league games against Clon.
At Ballymacarn Park, Ballynahinch claimed a hat-trick of home wins over De La Salle Palmerston as they ran out 13-6 winners.
In wet conditions, Aaron Ferris grabbed the game's only try and 'Hinch also had an eight-point kicking contribution from Ryan Bambry, who is fully recovered from a recent ankle injury.
For photos from the Ballynahinch v DLSP game, please click here.
Elsewhere, former Ireland Under-21 international Stuart Philpott notched Belfast Harlequins' only try in their 10-8 success over Bective Rangers at Deramore Park.
'Quins did not have it all their own way and after Kevin McKenzie nosed Bective in front with a 71st minute, the Ulstermen needed a late penalty from Keith Pyper to seal the win.
Powerful prop Martin Gately muscled his way over for a crucial try in Highfield's 13-3 Munster derby triumph over Thomond at Woodleigh Park.
Malone also picked up an opening day win. They claimed the scalp of Greystones by taking a 16-10 verdict at a cold Gibson Park, where Gareth Fry kicked three penalties and converted a penalty try.
Wanderers came away from Rosbrien with a valuable four points. A David Burns try and a vital conversion by former Shannon man Niall McNamara helped the Dubliners beat Old Crescent 7-6.
Arguably the match of the day took place at the RDS where Lansdowne and UCC shared the spoils in an entertaining 19-19 draw.
Gavin Dunne kicked four penalties and converted Scott Deasy's try for the Cork students, who are the reigning Division Two champions.
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