After James Garner left the series after the third season, (he's in one fourth season episode, 'Hadley's Hunters' that had actually been filmed for the third season), Warner Brothers thrashed around looking for a replacement from the actors they had under contract.
First they created a British cousin of the Mavericks, Beau, (actually he came for Texas just like Bret and Bart but was sent to Britain to be educated and learn a British accident), played by Roger Moore, late of 'The Alaskans'. He had the light comic touch the role needed and some of the scripts they gave him to do were pretty good, (so good they'd already been done on the studio's other western shows). Roger got an offer to return to England to do 'The Saint' and Warners again had to find a replacement.
They created a third Maverick brother "Brent" and assigned Robert Colbert, who had appeared several times in warner's shows and acquitted himself well. He was about the same size as James Garner and bore a vague resemblance from a distance, so they put him in Garner's old outfit, (which had been Errol Flynn's outfit in 'Silver River'). The public reaction, (and those in the other reviews here, was completely negative and after two shows 'Brent Maverick' disappeared and it became the Bart Maverick Show starring Jack Kelly.
I thought the Brent Maverick episodes were pretty good, better than several on the non-Brent episodes this season. I also found Robert Colbert a very agreeable performer. Like Jack Kelly, he lacked Jim Garner's bread smile and Oklahoma twang. He seemed less of a rogue at heart. (Garner would have made a great used car salesman.). He didn't have the precision of the stage-trained Kelly's performances. But he did have warm sincerity to his acting that I liked. I think if they'd continued with the character he might have grown on people. (It might have helped if they also made him a cousin or a nephew, not a never-before mentioned brother and given him his own distinctive outfit.)
Colbert got a second chance at series stardom a few years later in 'Time Tunnel', not a show that required warm earnestness. He final found a spot in the soaps, where such qualities were more useful.