Hundreds of veteran Port Authority professionals are about to be thrown from the gravy train, set to lose their longevity pay and other profitable perks.

As the agency strips these benefits in a cost-cutting move, a former high-ranking PA official says it will lead to a “serious brain drain.”

“You will see a mass retirement and exit,” the former official said.

Two top officials are already departing: Louis LaCapra, chief administrative officer, and Ernesto Butcher, chief operating officer. Each had a base salary in 2011 of $251,758. But LaCapra earned $350,405 including an extra $30,211 in longevity pay and $58,098 in cash instead of vacation time. Butcher got a $23,336 longevity payment and $24,208 in vacation pay.

In total, 24 PA vets got $10,000 or more in longevity pay in 2011 and 32 employees raked in $20,000 or more in vacation payments instead of time off.