In the Factory Service Manual at the top of the column you will see it say it is supposed to be measured in bar. But for my year and model that if you go down the numbers can't represent bar.

At idle with no abnormal leaks you should get at lest 22 mmHg. The seat of the pants telling point is if you do not have power brakes or after a pump on the brakes you don't have power brakes.

When the Check Valve that is built into the fitting that screws into the vacuum pump is not working your vacuum drops but you still have vacuum in the teens of mmHg.

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84 300D, 82 Volvo 244Gl Diesel