When the brakes are right it should darn near lock up all six tires with relatively little effort. I have read that to test the hydovac, place your foot on the pedal and start the truck. When the truck starts the pedal should go down slightly. THe other thing you should look for is if the pistons in a few of the wheel cylinders are stuck. If the the piston are stuck the pedal will feel firm put you will have very little brake action. I had a '64 rambler classic recently and my '49 6400 was easier to stop!