The Bulls made a mess of what looked like a negotiable job and their Super Rugby play-off hopes are now in doubt.
Playing at home, they were largely expected to do something against this modest Brumbies side that came into this clash on the back of five consecutive losses, but they made a complete cock-up.
To add salt to injury for the Bulls, two late tries for the Brumbies by Tom Banks and Andy Speight that settled this uninspiring match and ensured the much-needed victory, were scored with the Australians a man down after the red card to Folau Faingaa.
The Bulls, who travel to Hong Kong to meet the Sunwolves, welcome the high-flying Jaguares and travel to the Lions in their remaining matches after the June tests, have an outside chance of making it to the play-offs.
This is the second loss on the trot for them following last week's defeat by the Jaguares in Buenos Aires and coach John Mitchell will have to find a formula soon to motivate his players for the remainder of the round robin.
The Bulls got the scoreboard ticking with a try by flyhalf Handrè Pollard, who was released by a clever Johnny Kotze offload inside the opening five minutes and he found the middle of the poles from the conversion.
Pollard's conversion was his 50th point in the competition as he continued to make a strong statement to Springboks coach Rassie Erasmus for a starting place during the June tests against England in Johannesburg, Bloemfontein and Cape Town.The Bulls' lead was short-lived as their defence was caught in slumberland from the restart by alert Kyle Godwin who touched down next to the far-side flag and Christian Lealiiffano levelled with the conversion.
The Bulls were under pressure and they committed silly mistakes with Conraad van Vuuren penalised by referee Marius van der Westhuizen with a penalty that was put away by Lealiiffano for interfering with the Brumbies scrumhalf.
The home side responded shortly afterwards with their second converted try of the match by Roelof Smit, who benefited from clever interchanging play between Kotze and skipper Burger Odendaal as they retook the lead.
On the stoke of half time Lealiiffano missed a golden opportunity to bring the Brumbies to within a point of the Bulls when he missed a penalty.
The Brumbies assumed the lead three minutes after the restart through a try by Lechlan McCaffrey followed by a conversion by Lealiiffano but the Bulls levelled matters through a penalty by Pollard.
The Bulls pulled away once again when Kotze touched down next to the near-side flag but Pollard missed the mark for the first time on the afternoon.
Pollard recovered his scoring touch minutes later when he converted a penalty but the Brumbies scored their third try through Folau Faingaa with Lealiiffano converting to come within a point of the Bulls.
After the red card, Pollard scored a penalty but the Brumbies replied with two converted tries by Banks and Speight.
Bulls(14) 28
Brumbies(10) 38
Bulls - Tries: Handre Pollard, Roelof Smit, Johnny Kotze;Conversions: Pollard (2); Penalties: Pollard (3)
Brumbies - Tries: Kyle Godwin, Locky McCaffrey, Folau Faingaa, Thomas Banks, Andrew Muirhead;Conversions: Christian Lealiifano (3), Wharenui Hawera (2); Penalty: Lealiifano..