VALVE SEALS AND OR VALVE GUIDES.. IF IT IS AN OLD MOTOR YOU CAN BE SURE THE SEALS ARE AS HARD AS A WEDDING DICK..
CHECK THE VACUUM WITH A GAUGE.. IF IT IS BOUNCING COULD INDICATE A VLAVE TRANE PROBLEM (WORN GUIDES).... SEALS CAN BE REPLACE WHILE THE MOTOR IS IN THE CAR WITH THE PROPER TOOL.
DO...