AC/DC have been ramping up the promotion of their upcoming album, 'Rock or Bust.' And they haven't evaded any questions regarding two recent missing members: guitarist Malcolm Young, who had to step down after being diagnosed with dementia, and drummer Phil Rudd, who was arrested last week for threatening to kill someone (among other charges).

In a new interview with Rolling Stone, guitarist Angus Young says that his brother's health problems surfaced a few years ago, while the band was making its 2008 record 'Black Ice.'

“It’s something that had actually been happening for a long time,” he said. “[It] had surfaced even before the last project … [Malcolm] was still capable of knowing what he wanted to do. I said to him, ‘Do you want to go through with what we’re doing?’ And he said, ‘S---, yeah.’"

In fact, Malcolm had been receiving treatment during 'Black Ice''s tour. "He got good help, good medical care," Angus said. "But he was always a confident guy, and we made it work." He also pointed out that his brother "liked to finish what he started."

As for Rudd, Young added that the drummer had been showing some erratic behavior lately, like showing up 10 days late for the new album's recording sessions.

"One minute he was coming, then he wasn't, then he was," Young said. "We're not a band that likes to wait around." He eventually played on the album, but Young noted that he's "seen him in better shape. It was not the Phil we had known, after we had finished the last tour. He'd let himself go."

The band plans to continue on without Rudd for now. Young told Billboard that they will "go forward" without their longtime drummer. It seems like they were even prepared for this move. "Our problems had begun even before the situation he's in now," Young said. "And our thing was we were going forward."

'Rock or Bust' comes out on Dec. 2.

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