It could be going into the power unit.
if the m/c is dry there should be plenty of evidence of where it went. look on the inside of the tires at 6 clock position no signs of wetness you can bet the booster absorbed it. with it running and when you press on the brake does the tail pipe emit pretty blue smoke? if so that's the brakes fluid being drawn into the engine and burned. another way to tell is on a bendix type unit is to disconnect the vacuum line from the carb at the unit. if fluid runs out the unit needs repair or replacement.
