The Robins suffered just their second defeat of the season as Billy Paynter's 71st minute effort handed Vale the points to extend their lead at the summit.
Former Chelsea striker Knight has proved an astute signing by Brighton boss Steve Coppell and he bagged his seventh of the campaign to open the scoring against The Owls after eight minutes.
A tight contest ensued for the remainder of the game but the visitors' hopes of a comeback were extinguished on 83 minutes when Chris McPhee grabbed the Seagulls' second goal.
Barnsley and Swindon, who were a solitary point behind Port Vale at the start of the day, shared the spoils in a close encounter at Oakwell.
Sam Parkin put the visitors ahead on 20 minutes but Rory Fallon rescued a point for Barnsley with a 77th minute equaliser.
Queens Park Rangers needed a last-minute goal from veteran forward Paul Furlong to salvage a point from their trip to Luton.
On-loan hit-man Gary McSheffrey put The Hatters ahead on the hour mark but Furlong ruined Luton's day with a last-gasp leveller.
Hartlepool slid several places down the table after losing 2-1 at Brentford.
Ibrahima Sonko's 13th minute goal was cancelled out by Pools' Mark Tinkler before the break but Tommy Wright, on loan from Leicester, gave his temporary side the points just after the interval.
Bottom club Chesterfield were condemned to their fifth defeat in a row after being crushed 4-0 at Grimsby Town.
Stuart Campbell's first-half goal put Grimsby in control and second-half efforts from Graham Hockless, Marcel Cas and Mike Edwards settled the tussle.
The Spireites, who have yet to win this term, also finished the match a man light after Gus Uhlenbeek was sent off for foul and abusive language.
Plymouth Argyle stretched their unbeaten sequence to seven matches after twice battling back for a point at Wrexham.
Dennis Lawrence put Wrexham ahead, only for David Norris to quickly level the scores. Lee Jones's penalty restored the hosts' advantage but Tony Capaldi poached a point for the visitors on 49 minutes.
Bournemouth's good recent run continued after Warren Feeney's quick-fire brace in the second half secured a win over Rushden & Diamonds after the prolific Onandi Lowe had put the visitors ahead after just two minutes.
Wycombe Wanderers have not won since the opening day and their miserable spell of form continued after a 5-2 thumping at home to Oldham Athletic.
A Paul Murray double was the highlight for The Latics, who also scored through Daniel Hall, Calvin Zola Makongo and Chris Killen's spot kick. Andrew Bell netted both goals for Wycombe.
Managerless Stockport appeared to be coping with life after Carlton Palmer when Rickie Lambert gave them a half-time advantage at home to Blackpool.
But it was a tale of two halves as goals from Simon Grayson, Chris Clarke and John Murphy handed Blackpool the win, despite the inconvenience of Neil Danns's second red card of the season.
Colchester remain in a rich vein of form after a gritty success at Peterborough, which was earned through Joe Keith's last minute winner.
Craig Fagan's first-half goal was cancelled out by Neil Wood but The Posh also had a men sent off after Simon Rea received two yellow cards.
Notts County and Tranmere Rovers shared four goals at Meadow Lane with the visitors still searching for their first win since the opening day.
Mark Stallard put the home side ahead on four minutes but Simon Haworth's brace gave Rovers the lead before Clive Platt pulled a point of the bag for The Magpies.
Click below for an individual match summary:
Barnsley 1-1 Swindon
Bournemouth 2-1 Rushden & D'monds
Brentford 2-1 Hartlepool
Brighton 2-0 Sheff Wed
Bristol City 0-1 Port Vale
Grimsby 4-0 Chesterfield
Luton 1-1 QPR
Notts County 2-2 Tranmere
Peterborough 1-2 Colchester
Stockport 1-3 Blackpool
Wrexham 2-2 Plymouth
Wycombe 2-5 Oldham
