<html><body><div style="font-family: Arial; font-size: 12pt; color: #000000"><div>re: ' ... when stopped at a light….idling ,foot on brake……the idle increases by about 300-500 rpm… is this normal?'<br></div><div><br></div><div>Nope. When you step on the brake pedal, a slave cylinder in the servo moves a 'T' valve which vents atmospheric pressure air into the large 'vacuum' canister, pressing on a large piston which works against below-atmospheric pressure--i.e. vacuum--on the other side (in normal running, vacuum is applied to both sides of the piston). If your idle increases when you apply brakes that means you have an air leak somewhere in the system. I can only speculate where the leak(s) might be originating, but my first guess would be the around the seal on the vacuum/air piston in the canister. Since the seal is a leather strip bolstered by a large rubber O-ring--both of which are known to dry out and crack or even disintegrate--atmospheric air could get past the seal into the vacuum half of the piston, which vents directly to the intake manifold (through the check valve). The check valve--which, I believe, is there to permit one final application of boosted brakes should the engine quit--would, in theory, prevent that from happening, but they can fail as well. An alternate cause might be the seals on the T-valve, which should close vacuum input when the brakes are applied, but if those seals have failed it might allow atmospheric air into the system.<br></div><div><br></div><div>A failed check valve--it's a flimsy reed-type valve--would not cause this problem, but a working valve might mitigate the high idle. At any rate, you need to fix the cause(s) of the problem (see above), and if your check valve has failed you need to replace it as well. Unfortunately, I don't know of any source for new replacements, but I was able to get a good used one from BCS many years ago. My original valve had apparently failed in the past, and a PO had installed a plastic replacement on the side of the wheel well, which worked fine but looked wrong.<br></div><div><br></div><div>Servo rebuilds seem to be hit-or-miss; I rebuilt mine many years ago and it's worked perfectly since. There is a dry lubricant coating on the inside of the vacuum canister which prevents the leather/rubber seal from sticking; I spent quite a few bucks to get a proper coating--I think it's molybdenum-based--and applied several coats. I wouldn't be surprised if the PO skipped this step, possibly causing premature wear on the leather/rubber seal.<br></div><div><br></div><div>Bob<br></div><div><br></div><hr id="zwchr"><div style="color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><br><div><br></div><style><!--
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--></style><div class="MsoNormal"><p class="MsoNormal">Hello,</p><p class="MsoNormal"> </p><p class="MsoNormal">I have the original servo on my 1966 BJ8 (Girling MK2A) it was rebuilt by PO at some time</p><p class="MsoNormal">I recently noticed when stopped at a light….idling ,foot on brake……the idle increases by about 300-500 rpm… is this normal? Also noticed the brake peddle is a little lower when applied. </p><p class="MsoNormal">The car appears to stop …OK</p><p class="MsoNormal">What tests can I do ?</p><p class="MsoNormal">Can I put a new check valve in vacuum line ……without removing the valve on the servo? Or would that cause a problem?</p><p class="MsoNormal">And yes I looked in the archives…….little confusing</p><p class="MsoNormal">Any help would be appreciated</p><p class="MsoNormal"> </p><p class="MsoNormal">Cheers</p><p class="MsoNormal">Mitch</p><p class="MsoNormal"> </p><br></div><div><br></div></div><div><br></div></div></body></html>