With the play-off challenge over, Johnson dropped all of his loan players and handed starts to several Academy products.
Teenage defender Jack Cowgill - on his full debut - was at fault to gift Posh the opener in the first half, Conor Washington pouncing on 16 minutes.
But the Reds battled back, defying their inexperience and should have been level by half-time.
Cowgill, Mason Holgate and Luke Berry all impressed while Conor Hourihane spurned an open goal.
Posh were reduced to 10 men when Kgosi Ntlhe was dismissed for two bookings.
And in the fifth minute of stoppage time, up popped Berry to head home Hourihane’s corner for his first Football League goal.