Two second half strikes for The Bees, courtesy of Fredrik Hammar and Dru Yearwood did the damage in a match where Neil MacFarlane’s side created a number of chances, particularly in the opening 45 minutes.
The Brentford B coaching staff had a number of first teamers involved in the match day squad with Jan Zamburek, Luka Racic, Patrik Gunnarsson, Halil Dervişoğlu and Dru Yearwood all featuring against The Blues.
It was the home side who had the first sights of goal with a couple of attempts from distance and it took until the 17th minute for The Bees to create their first opportunity, which they almost went ahead from.
As Jan Zamburek did well to cut in from the left-hand side, the ball found its way to Matej Majka who shot just over from six yards out.
Billy Gilmour would have another effort at goal for The Blues when he struck powerfully from the edge of the box but for Patrik Gunnarsson to collect and hold well on 26 minutes.
Brentford B then almost fell behind in unfortunate circumstances when a long ball forward into the Brentford back line was glanced back by Japhet Sery Larsen, but it evaded Gunnarsson and came back off the post before the keeper scrambled back to push the ball away off the line.
The impressive Mads Roerslev, who threatened down the right wing throughout, almost got The Bees ahead when Nick Tsaroulla’s cross was helped on by Dervişoğlu before he volleyed at the far post straight at Nicholas Tie in the Chelsea goal.
The Bees started the second period the better side and had another glorious chance when Zamburek controlled well before whipping a curled left footed effort against the Chelsea post as the defenders managed to get back to clear the rebound.
But that pressure paid off as Brentford B went ahead on 56 minutes. As Roerslev darted down the right he crossed well, and once again Dervişoğlu made a nuisance of himself as he prevented the clearance as Hammar turned well to volley home into the bottom right-hand corner to make it 1-0.
A much-changed Chelsea side in the second period then began to create more chances for themselves with Thierno Ballo beginning his own personal duel with Gunnarsson. As the striker made a clever run through the centre he controlled well before unleashing a low effort at goal which Gunnarsson saved well before racing to his feet to prevent a follow up effort from the Chelsea attackers.
Ballo would have another attempt 10 minutes from time as he received the ball just on the edge of the box before working himself some space and shooting at the legs of the Icelandic shot stopper.
But as Chelsea looked to level in the latter stages it would be The Bees who would double their lead when substitute David Titov made a clever run on the overlap down the left before he slotted the ball low into the box for Yearwood to side-foot home to make it 2-0.
The last of the action would see Ballo attempt to pull a goal back when the ball found him in the Brentford box, but from a narrow angle to the right, his shot was pushed away to safety as the Bees picked up a hard-earned win.
Brentford B: Gunnarsson, Roerslev, Tsaroulla (Titov, 60), Yearwood (Carre, 83), Racic, Maghoma, Majka (Pressley, 60), Hammar (Adams, 60), Zamburek, Dervişoğlu (Tue Jensen, 75), Sery Larsen (O’Connor, 60)Subs not used: Andersson, Mitchell, Koutsimouka