If the brake pedal goes down low after you start, you may have a torn
diaphragm inside of the booster. All the vacuum lines ok? Can you hear
the booster hiss when you hit the brake while the vehicle is running?

--
Jackson

Two things I would look for...

1. Does the fluid level go down any if you press and hold the brakes down
multiple times? If so, you have a leak somewhere. It may be a bad caliper
at one of the wheels, but a car of that age could have rusted brake lines
too. I recently had a similar problem, and couldn't figure out where it was
leaking. After tracing the brake lines, I discovered the brake lines in my
1976 Rabbit run through the passenger compartment and had rusted through
under the carpet. So I had to replace the brake lines and clean up a mess
of brake fluid in the floorpan. :)

2. If you press than hold the brake pedal down, does it slowly drop to the
floor over time? If so, you could have a bad master cylinder. Basically,
the seals in the cylinder are letting fluid leak by, and the pedal drops
accordingly.

But, you description seems to indicate the brakes are fine until you start
the engine. Which leads me to think that's just the normal operation of the
vaccuum brake booster.

You might check the freeplay on the brake pedal, but if the brakes work
well, and you aren't losing any brake fluid, I don't think there's any
problem.

Anthony