Mr. Beberman drives the last real seltzer truck in New York, a wooden-slatted affair with crooked racks and side doors that are stuck open — the easier to strap the worn wooden cases to the side.

For nearly 40 years, he has delivered seltzer in thick, old siphon bottles to thousands of Brooklynites, each customer receiving a case of 10 every other week for $25, cash.

But on Sept. 15, just before the start of the Jewish High Holy Days, one of the busiest times of the year, Brooklyn’s Gunga Din of soda water went down, and now several hundred customers are resorting to rationing or even privation.

“The first couple of days, I was drinking it slow and making it last,” said Joshua M. Bernstein, 31, of Crown Heights, a food writer and a customer of Mr. Beberman’s for three years. “We’re down to our last bottle, so we’re saving it for a special occasion.”

Matt Levy, 29, a tour guide and fourth-generation Brooklynite, said he was rationing the case that Mr. Beberman delivered to him in Bushwick two weeks ago. He had five bottles left.

“We’re in a seltzer drought, and we have to prepare for it,” Mr. Levy said. “We have one of these little seltzer-making machines at home, but nothing is as good as Ronny’s seltzer.”