How John Lennon's Ex May Pang Helped Heal His Relationship with Son Julian (Exclusive)

When John Lennon first began dating May Pang, he hadn't seen his son Julian in several years.

But with Pang's encouragement, the father-son relationship began to heal, something Pang remains proud of, she tells PEOPLE in this week's issue.

"He hadn't seen [first wife] Cynthia in years," Pang, 72, recalls. "[But] I said, 'I'll be there to help guide this along.' And he was so appreciative."

Pang was the personal secretary to Lennon and his wife Yoko Ono when she first met the young Julian at the couple's Tittenhurst Park estate in England. As she explains in her new documentary The Lost Weekend (out in theaters April 13), the two forged a fast bond, playing together in golf carts.

By the time Pang started dating Lennon in 1973, she took things a step further, convincing the former Beatle to invite Julian and his mother Cynthia to visit for the first time in years. When Pang and Lennon — whose 18-month romance he later dubbed his "lost weekend" — moved in together in 1974, they made sure their Manhattan abode had a bedroom set aside for Julian.

"When Dad was with May, he always seemed very happy and youthful," says Julian, 60. "She brought a light to our relationship."

RELATED VIDEO: Julian Lennon Says He 'Always Hoped' He'd Get to Place of Peace After Forgiving Dad John Years Ago

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The couple brought Julian to Disney World, and also to Montauk, where they had their hearts set on a Scottish-style home. She says there were plans in motion to buy the house before Lennon went back to Ono in 1975.

Pang adds she "was glad that we were there" for him — and reveals that although her romance with Lennon didn't last, she did find a lifelong friend in Cynthia Lennon that would last until her death in 2015.

"She was great. I'd do anything for her," Pang says of Cynthia, whom she would see regularly over the years.

She and Julian have also maintained contact over the years, and last year, he used a childhood photo taken by Pang as the cover of his most recent album Jude. "I'm glad he's his own person," she says. "That's what I like."

The Lost Weekend hits theaters on April 13. Pang will also be hosting two photo exhibitions at City Winery in New York City (April 8 and 9) and Philadelphia (April 15 and 16).