People who are helping diagnose certain brake problem conditions - in particular a hard brake pedal - often suggest that you measure the vacuum pressure. The advice will sometimes mention that your engine must produce more than 18” of vacuum to give that nice easy pedal feel. In fact, we just provided that explanation in our recent Teck Talk article on diagnosing causes of a hard brake pedal.
So you’ve measured your engine vacuum and it is less than the desired amount. This may explain the hard pedal, but what is causing the low vacuum problem? While cruising around the internet recently, we found a great article posted up by the folks at onallcylinders.com. In the article they provided a really slick chart that explains what your vacuum gauge is telling you.
Based on their story, step one is checking to confirm that all vacuum hoses are connected and not leaking. To begin measuring, connect the vacuum gauge to an intake manifold vacuum source. You can either add a tee-fitting into an existing vacuum source or alternatively, disconnect one of the vacuum lines like the hose that leads to your transmission. Once your engine has warmed up to operating temperature, you can begin reading your vacuum gauge.