Aircap:

If your brake fluid was low you should have seen a glowing brake light on the dash. Check your master cylinder and the floats inside to ensure the contacts are good and the light works. You would also be wise to bleed the brakes if the fluid got low enough for the pedal to bottom out. A power booster has been known to conceal a spongy pedal. Also, I would consider checking and/or replacing the check valve in the PB vacuum line. If there was a vibration it might be the source; maybe pulsing because of the low fluid level and the long pedal stroke.

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