Brake booster vacuum testing

rosehonda, Mar 24, 10:16pm
I tested my brake booster with a vacuum pump using these instructions. I suspect a vacuum leak due to slight roughness of the idle.
http://ssbrakes.com/attachment/85569-Vacuum%20Brake%20Booster%20Testing%20and%20Diagnosis.pdf

I drew a vacuum of 20hg and let it sit for 5 minutes. In 5 minutes I lost about 1.5 - 2hg max of vacuum.

When pressing the peddle (step 3) it left some in reserve so that checked out fine.

When holding down the peddle for 30 seconds i lost 2hg - 2.5hg max of vacuum.

Check valve seemed fine. My question is Are these 1 or 2 hg losses of vacuum during testing enough to cause a problem or are they only minor? Not that keen on replacing the booster considering the cost of it.

moparpete, Mar 24, 11:05pm
Should be o.k. Would pay to get it checked at a reputable brake specialist workshop. I`ve dealt with Apex brake and clutch in Hamilton, and have had good results from them.
http://www.apexautocentre.co.nz/8/contact-us

frytime, Mar 24, 11:32pm
Follow rubber hose to manifold and block there. Will prove your leak from booster if idle settles down. Not very common for boosters to fail tho

intrade, Mar 25, 12:01am
ok i did not read these distructions as i know how to eliminate a vaccume booster for a problem on rough idle.
You simply clamp off the hose to the booster with vice grips and some rubber hoses on the nedle nose vice grip to not damage the supply hose with the jaw.
This eleiminates the booster and if your 99% sure that the problem is on what ever is connected after the point you clamped off. if the engine runs good with it clamped off and rough with no clamping.
Most common place for vaccume leek is rubber conduit after mass airflow intake mainifold worping it plastic junk or the positive crank ventilation system pcv valve .These arte the most common ones

intrade, Mar 25, 12:09am
might be good to know what car we are talking here.

sr2, Apr 14, 6:18pm
You're fine with those figures, your booster is not loosing vacuum there is no need to have it checked.

As a general rule of thumb modern Hydrovac style boosters seldom ever leak in the static position. A perforated diaphragm, (more often than not caused by a leaking master cylinder) will cause a loss of vacuum only when the unit is cracked i.e. the pedal is pushed.

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