To me, Bart seemed clear-headed, not whacked out on medication for his wound. So he was apparently in his right mind when he inferred that he had no other brothers except for Bret.
And yet Brent Maverick, as played by Robert Colbert in two 'Maverick' episodes, was presented as one of the Maverick brothers on the show. In one of those episodes, "The Forbidden City", Jack Kelly also appeared. And he probably introduced Brent as his brother. There's also the publicity picture still floating around of Kelly and Roger Moore in costume as Bart and Beau with Colbert as Brent (looking like a poor man's Bret... a wooden, robotic Bret.)
Had Brent tried to pass himself off as a Maverick without official support, Toobworld Central was ready to play it out four different ways.  One, that if he was a Maverick brother, he was Beau's brother.  He wouldn't need a British accent - Beau picked that up as an affectation after spending a decade in England, banished for an indiscretion during the Civil War.  (He became a war hero.)The second splainin was that he was passing himself off as a Maverick, but was in fact Cherokee Dan Evans, the character he played in the episode "Hadley's Hunters" (one of the greatest contributions to TV crossovers ever.)
The third option would be to claim that he was an android similar to Hymie the Robot from 'Get Smart'. (Dick Gautier - now there's an actor who could have pulled off that 'Maverick' mojo!) But O'Bviously we'd never fight for that pozz'bility. Otherwise we're wandering into the territory of "The Wild Wild West Revisited". Blerg.
Finally, and the most enticing to a fan of the TV Universe mosaic, he was actually Dr. Doug Phillips, a time-traveling researcher at the top-secret Project Tick-Tock underneath the Arizona desert, as s
een in 'The Time Tunnel'.  For just those two episodes he had been separated from his associate Dr. Tony Newman and so was hiding in plain sight until they could be reunited.  To get Bart to go along with the idea must have cost Dr. Phillips something in return....But sadly, Brent was presented as a son of Beauregard "Pappy" Maverick and that puts the kibosh on all those theories, while at the same time it presents a Zonk with Bart's statement that he and Bret were the only brothers.
But it's easily splained away: Brent is a half-brother to Bret and Bart, the product of one of Pappy's indiscretions. Talk about a "Seed Of Deception"! And so, at the time of the episode, the Maverick boys didn't know they had a brother.
Easy peasey.....
BCnU!
 JOHN F. PARKER
But he didn't spend all those intervening years in Washington, D.C. He traveled the West, where Edwin Booth finally caught up with him with the intent to kill him.....


Everything else about Central City has that California feel, so that's where we're going to say it is. And with that, we're going to claim a geographical link between the Mavericks Bret and Bart with Barry Allen! 
BCnU!
AS PLAYED BY:







BCnU!

For example, here are two bumblers who tried to pass themselves off as Frank and Jesse James only to run afoul of a 20th Century reporter and a stranded Martian.....



SAMUEL CLEMENS
AS PLAYED BY:
Another reason is the timeline for the entire series. Unlike other shows which are usually set in one location, 'Maverick' traveled all over the Western frontier. That takes time to get from one place to another. Unlike 'Kojak', it's unrealistic to think an episode of 'Maverick' can take place a day later than the last one, or even a week later if that's the way a detective show or medical drama wanted to play it.
Miss Lola Fairmont

By the end of "Seed Of Deception," yet another Doc Holliday impersonator had shown up in 
Eventually the real Doc Holliday returned, this time to Tombstone, and Jim Mundy snuck back to his ranch near Bonita, California. There he finally met his match in the Maverick brothers and was caught red-handed trying to rob the town's bank.
BCnU!
BCnU! 
HORACE GREELEY

