Somewhere I read that vacuum reading at the main line should be between 21 and 23 inches hg. I first tested my vacuum several months ago and (at idle) it would creep up almost to 20 before dropping back below 10 then slowly creep upward again. I have since rebuilt the vacuum pump, now the reading is steady but is only around 16 inches when car idles. Increasing throttle does increase vacuum reading but it does not go to 20. I disconnected vacuum line from brake booster, capped the end and then took the reading - it was 21, so it seems that the vacuum pump is working well. I never seem to have problem with brakes, but was wondering if the booster could have enough of a leak to affect vacuum reading without really affecting braking.

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Dave
1995 C280
1999 Triumph Daytona 955I - my speed fix.
1982 300TD - Gone, but not forgotten.