First, if your brake fluid disappears without an obvious wet spot on the rims the most likely place is inside the vac cylinder. There is only one seal that keeps the fluid from entering the vacuum cylinder via the hydraulic piston.
The test for a working vac while on the vehicle is simple. Start the motor and let it warm up. At idle, press the brake pedal a few times to evacuate the cylinder or air and hold the vacuum (you want to do this at idle as you should have 15-20in vacuum available.
Turn off the motor.
Now, if everything is working the vac canister should hold the vacuum. With the motor off, you should get AT LEAST 2-3 hard pedal brake applications before the brake pedal gets soft. Each time you hit the brakes you are letting in atmospheric air and depleting the stored vacuum.
Any other result means time to bleed the lines or take off the vac for testing. Don't forget to test/inspect the vacuum check valve (usually mounted on the firewall or near the intake manifold vacuum port.
I have a manual that shows SERIES I and II vacs and testing/troubleshooting. Its
HERE. I have several SERIES III vac manuals but not online.. the pluming and operation is the same... just the internals are slightly different.
