Thomas,
"Spongey pedal" could be air in the system but you said nothing about brake problems other than while stopped at a light. The pedal going to the floor while stopped indicates an internal leak in the master cylinder since you said there was no loss of brake fluid.
If the booster or its' vacuum hose fails, you get a hard pedal that requires a lot of pressure to stop, not a spongey pedal.
A vacuum leak at the booster will cause a lean mixture to one or more engine cylinders, like any other major vacuum leak. (I just replaced the failed booster on my '72 250. It caused a hard pedal, much higher pressure to stop at all times and a miss from # 6 cylinder)
I would probably bleed the brakes first to see if there's an improvement. If not, it should take less than an hour to replace the master cylinder.
As for the mechanic, he should have checked the car out AFTER he did the work.

Happy Motoring, Mark

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DrDKW