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re: ' ... when stopped at a light=E2=80=A6.idling ,foot on brake=E2=80=A6=
=E2=80=A6the idle increases by about 300-500 rpm=E2=80=A6 is this normal?'=
=20
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' c=
anister, pressing on a large piston which works against below-atmospheric p=
ressure--i.e. vacuum--on the other side (in normal running, vacuum is appli=
ed to both sides of the piston). If your idle increases when you apply brak=
es that means you have an air leak somewhere in the system. I can only spec=
ulate where the leak(s) might be originating, but my first guess would be t=
he 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 k=
nown to dry out and crack or even disintegrate--atmospheric air could get p=
ast the seal into the vacuum half of the piston, which vents directly to th=
e intake manifold (through the check valve). The check valve--which, I beli=
eve, is there to permit one final application of boosted brakes should the =
engine quit--would, in theory, prevent that from happening, but they can fa=
il as well. An alternate cause might be the seals on the T-valve, which sho=
uld close vacuum input when the brakes are applied, but if those seals have=
failed it might allow atmospheric air into the system.=20
A failed check valve--it's a flimsy reed-type valve--would not cause this p=
roblem, 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 valv=
e 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 fro=
m 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.=20
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 o=
f 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 skippe=
d this step, possibly causing premature wear on the leather/rubber seal.=20
Bob=20
----- Original Message -----
Hello,=20
I have the original servo on my 1966 BJ8 (Girling MK2A) it was rebuilt by P=
O at some time=20
I recently noticed when stopped at a light=E2=80=A6.idling ,foot on brake=
=E2=80=A6=E2=80=A6the idle increases by about 300-500 rpm=E2=80=A6 is this =
normal? Also noticed the brake peddle is a little lower when applied.=20
The car appears to stop =E2=80=A6OK=20
What tests can I do ?=20
Can I put a new check valve in vacuum line =E2=80=A6=E2=80=A6without removi=
ng the valve on the servo? Or would that cause a problem?=20
And yes I looked in the archives=E2=80=A6=E2=80=A6.little confusing=20
Any help would be appreciated=20
Cheers=20
Mitch=20
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<html><body><div style=3D"font-family: Arial; font-size: 12pt; color: #0000=
00"><div>re: ' ... when stopped at a light=E2=80=A6.idling ,foot on brake=
=E2=80=A6=E2=80=A6the idle increases by about 300-500 rpm=E2=80=A6 is this =
normal?'<br></div><div><br></div><div>Nope. When you step on the brak=
e pedal, a slave cylinder in the servo moves a 'T' valve which vents atmosp=
heric pressure air into the large 'vacuum' canister, pressing on a large pi=
ston which works against below-atmospheric pressure--i.e. vacuum--on the ot=
her 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 lea=
k(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 o=
ut 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 mani=
fold (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 w=
ell. An alternate cause might be the seals on the T-valve, which shou=
ld 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 cau=
se 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. Unfortuna=
tely, I don't know of any source for new replacements, but I was able to ge=
t a good used one from BCS many years ago. My original valve had appa=
rently 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><d=
iv><br></div><div>Servo rebuilds seem to be hit-or-miss; I rebuilt mine man=
y 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/ru=
bber 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=3D"zwchr"><div style=3D"color:#000;font-weight:normal;font=
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--></style><div class=3D"MsoNormal"><p class=3D"MsoNormal">Hello,</p><p cla=
ss=3D"MsoNormal"> </p><p class=3D"MsoNormal">I have the original servo=
on my 1966 BJ8 (Girling MK2A) it was rebuilt by PO at some time</p><=
p class=3D"MsoNormal">I recently noticed when stopped at a light=E2=
=80=A6.idling ,foot on brake=E2=80=A6=E2=80=A6the idle increases by about 3=
00-500 rpm=E2=80=A6 is this normal? Also noticed the brake peddle is =
a little lower when applied. </p><p class=3D"MsoNormal">The car=
appears to stop =E2=80=A6OK</p><p class=3D"MsoNormal">What tests can I do =
?</p><p class=3D"MsoNormal">Can I put a new check valve in vacuum line =E2=
=80=A6=E2=80=A6without removing the valve on the servo? Or would that=
cause a problem?</p><p class=3D"MsoNormal">And yes I looked in the archive=
s=E2=80=A6=E2=80=A6.little confusing</p><p class=3D"MsoNormal">Any help wou=
ld be appreciated</p><p class=3D"MsoNormal"> </p><p class=3D"MsoNormal=
">Cheers</p><p class=3D"MsoNormal">Mitch</p><p class=3D"MsoNormal"> </=
p><br></div><div><br></div></div><div><br></div></div></body></html>
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