Another activist hedge fund has joined the raid on the Brink’s Company. Steel Partners on Tuesday warned the security company famous for its armored trucks to seek a spin-off or sale, or else. It also disclosed a 6.2% stake in Brink’s.

“Steel Partners II believes that significant shareholder value would be unlocked if the issuer were to pursue a tax-free spin-off of one of its two business segments,” the hedge fund wrote. Steel further demanded an immediate sale of Brink’s if it does not pursue a spin-off, and called on the company to increase its stock buyback plan fivefold to $500 million.

Steel is the third activist hedge fund to express its unhappiness with Brink’s’ efforts to boost its stock price. Both Pirate Capital and Millbrook Capital Management have sought a tax-free spin-off, with the latter nominating a slate of board candidates for the company’s 2008 annual meeting. In November, Richmond, Va.-based Brink’s said it had retained the Monitor Group to investigate strategic alternatives for the company.