The actor's daughter, Jenny, tells TMZ ... Al passed away Friday in Las Vegas as a result of his Alzheimer's battle. She tells us that before he got into acting in the '90s, he was a 29-year-old Air Force vet who served two tours in Vietnam ... so he served honorably.

Jenny also says Al loved fans of his and was happy to stop and chat with anyone who might've recognized him from the show over the years -- noting, he loved his job.

Al will best be remembered for playing Col. Stan Valchek in the hit HBO show from the 2000s. In the series, he was a high-ranking Baltimore PD official, often portrayed as somewhat corrupt ... who'd give the main character cops a hard time in their investigations.

Valchek was a recurring character in all five seasons of "The Wire," but played a major part in Season 2 ... when he was dealing with dock workers who'd gotten mixed up with the Greek mob. His character was portrayed as a proud Polish-American -- a running gag throughout.

Great role, and Brown played it well for years. In addition to his work on HBO ... he also guest starred in other shows like "Rescue Me," "Forensic Files," "Law & Order: SVU," "Commander in Chief," "Maron," "The Hustler" and others.

Al even had a few notable movies in his repertoire ... playing small roles in "The Replacements," "12 Monkeys," "Lay the Favorite," "Red Dragon," "Liberty Heights," and more. He had a knack for playing hardboiled cops and law enforcement types ... and he did 'em justice every time.

He's survived by his children and extended family. Al was 83.