For years, Brooklyn commuters have opened their wallets for Robert McMahon, handing cash to this heroic and heartbreaking figure, a Vietnam vet in combat fatigues, his left arm missing and his right leg crippled, as he panhandles on Ocean Parkway in Kensington.

He plays to their patriotism, having scrawled his nickname, “Rambo,” on the back of his camouflage jacket, along with his years of service with the Marines and two stints in ‘Nam that saw heavy action. The top of his empty left sleeve is pinned to his uniform shoulder, and he drags his bum leg behind him.

When drivers stop for red lights, McMahon, 53, hobbles over and salutes gallantly, juggling a paper cup and a cardboard sign that reads, “Vietnam vet.” They give freely.

They are being scammed.

McMahon has two arms — and was seen using them last week to count the wads of cash he took off kindhearted New Yorkers.

He is not crippled, as was readily apparent when he swiftly dodged inquiring Post reporters.

And it seems he never served in the Marines nor in Vietnam, according to Corps and Veterans Administration officials who could not find any record of him.

But he carries on with his wounded-soldier act, day after day, weaving through traffic and occasionally cursing out people who refuse to give. He’s been at it since at least 1987, when he pretended to have a missing leg and pushed himself around in a rusty wheelchair, according to a friend.

Tuesday, The Post spotted the bearded, one-armed bandit at 4:30 p.m. at Ocean and Foster Avenue, where he limped over to the windows of idling vehicles and held up his cup.

Many gave dollar bills. He yelled an expletive at a driver who turned him away.

Some gazed at the back of his jacket, which claims he served in Vietnam from 1972 to ’73.

At 5:09 p.m., McMahon quit for the day, darted across Ocean Parkway’s seven lanes with surprising agility and walked briskly east along Foster.

Two blocks later, McMahon adjusted himself and quickly thrust his left arm out from under his jacket. He used both hands to count his cash.

When he realized a photographer was taking his picture, McMahon tried to hide his left arm by putting it behind his back, then spat and screamed obscenities at the lensman, threatening to kill him. “I’ll put a bullet in the back of your head,” he howled.

The ugly scene was not unfamiliar to police and prosecutors.

They say McMahon is a menace with a rap sheet of 20 arrests, many for disorderly conduct and “fraudulent accosting” — legalspeak for scamming victims on the street. His most recent collar came on May 27, when 66th Precinct cops found him blocking traffic at Ocean Parkway and Beverly Road at 12:50 p.m.

Wednesday, a reporter posing as a passer-by asked how he lost his limb. He winced in mock pain: “I don’t like to talk about it.”

Asked about his service, he claimed he was with the Marines’ 2nd Division and fought in Vietnam in 1972 and ’73.

Robert Gainey, a 68-year-old vet who did two tours in Vietnam as an explosives expert, has known McMahon for 23 years — and doubts he was in the war.

“Us guys who are still around, we can spot each other right away,” he said. “We’ve talked about the war. And he’s said things to me that you can read in a book or see in the movies. He didn’t say anything to convince me he was really there.”

Gainey said he first spotted McMahon in 1987 in a wheelchair on the Manhattan side of the Brooklyn-Battery Tunnel, pushing himself among motorists and begging for money.

“He had his leg tucked up underneath him,” said Gainey. “He was pretending he lost it. Later I saw him around here and said, ‘Oh, this guy’s from Brooklyn.’ ”

The leg apparently recovered. But then the arm went.

“He’s been doing the fake-arm thing for at least 20 years,” said Gainey. “He’s crazy as a bat. Everyone around here knows about it.”

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 (Source: NY Post)