Brakes on 1968 280SL.

This morning while waiting at a stoplight, the brake peddle became soft. I pumped to regain braking. I drove home immediately and checked brake fluid and wheel cylinders for leaks. None detected.

Is the Brake Booster the culprit? Vacuum booster hose to the intake manifold? The one man shop that typically services the car is closed for a couple of months. The owner is visiting Germany.

I can take it to another shop, but I'd like some ideas before I trust a strange shop.

Thanks,

Meauxhawk