After a even first half, Neil MacFarlane's side had Young-Coombes to thank for opening the scoring on the hour. It was a quick-thinking rebound from Jefferies soon after that saw the B Team double the advantage, just before Young-Coombes sealed the win a few minutes later.
Joel Valencia got 45 minutes under his belt, starting the game in an eleven that showed wholesale changes from the last outing at Charlton. Six players missed out through international commitments, while Jude Russell returned to action after a spell out injured.
Barnet fashioned the first real chance after 12 minutes, cutting in from the right wing to strike a ball towards Ben Winterbottom. The keeper caught it with ease but had to be sharp a few minutes later when called into action again. This time, he got down smartly to his right and palmed the ball behind.
The first half was largely a midfield battle and it was rare to see the ball drift into the final third. It almost worked out nicely for the home side after 25 minutes, Valencia squaring the ball back along the edge of the area but narrowly behind Jefferies racing in for a chance to strike it.
Young-Coombes had his head in his hands with 34 on the clock. Lewis Gordon started the move on the left wing, passing the ball inside to Ryan Trevitt. He then found Young-Coombes just inside the box with no one around him, the forward pivoting before firing low towards goal. The shot lacked the power it needed to trouble the goalkeeper and another chance went untaken.
Trevitt was involved again just before the break. He latched onto a pass by Jefferies, went to around the keeper and was chopped down only for the assistant to raise his flag on the far side. A narrow escape for Barnet as they went into the interval all square.
At half-time, MacFarlane swapped out Valencia for Max Haygarth and also made a change in goal. Roco Rees replaced Winterbottom for the second period.
It took just five minutes for Brentford to start creating better chances. Trevitt took aim and fired from the edge of the box, bringing a fine save from Aymen Azaze. The keeper tipped the ball onto the crossbar and it bounced away from danger.
With an hour gone, two more subs for Brentford came into the fray. Luka Racic and Ben Hockenhull replaced Tristan Crama and Nico Jones in the back three. Five minutes later, it was one-nil.
Jefferies started it, clipping a ball to Trevitt on the half-turn in the middle of Barnet's half. He slipped a perfect pass into the path of Young-Coombes, who rounded the keeper and tucked it home.
Just three minutes later it was two. This time Haygarth played the killer pass to Trevitt, who took a touch before striking at goal. It was parried but only as far as Jefferies, who volleyed the ball low into the back of the net.
With 75 gone, Young-Coombes thought he'd grabbed his second. This time it was ruled out for offside, Jefferies pass just coming a fraction too late for the forward. He finished it with some style regardless of the outcome.
He didn't have to wait long for another chance. Quality pressing work from Gordon and Trevitt on the left meant the ball dropped to the trialist to knock forward. It fell nicely for Young-Coombes, who shrugged off his defender before slamming a low shot through the legs of the keeper and in to make it three-nil.
He might have made it four with five minutes to go, darting onto a through-ball into space behind the back four. Instead, his shot found the side netting via a deflection from a recovering defender.
The visitors had the ball in the net after 86 minutes but it was disallowed for offside. A quick counter-attack that ended in a long-distance effort, replays showed the initial pass had actually been onside. A lucky escape for MacFarlane's men right at the end.
There was still time for an attack from the home side. Wraynel Hercules, running riot down the right all afternoon, broke into the box and smashed a ball across the face of goal. It narrowly avoided the attackers running onto it and sailed wide of the far post.
Brentford B: Winterbottom (Rees h/t); Russell, Crama (Hockenhull 60), Jones (Racic 60); Hercules, Jefferies, Trialist, Gordon; Valencia (Haygarth h/t), Trevitt, Young-Coombes