Subject: [Mgs] Clutch problems wuth thermal component

From: David Councill via Mgs <[email-masked]>
Date: Mon, 7 Nov 2016 08:13:29 -0800

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I'll ask the question before I start any serious disassembly. My near daily
driver, 1964 MGB, is still on the road although the next few months I will
likely drive it a lot less as darkness descends and roads start getting icy.
A month ago, I had seal failure on my clutch master cylinder, resulting in
the puddle of hydraulic fluid near my feet. Upon disassembly, I saw some
pitting in the bore which caused premature seal failure (I rebuilt this unit
about 5-6 years ago, not sure if I missed the pitting). So I decided to
replace the master cylinder with a new Lockheed unit. Now I have a new
problem - pedal motion is perfect until the car warms up and then most of
the pedal resistance is lost briefly. Typically, when I drive to work (or
leave work), the car starts out cold, warming up on the short city drive,
and then I have 8-12 miles of highway driving. The first time I use the
clutch after the highway drive, the pedal goes almost to the floor with no
resistance but enough to still work. But once I do that one time, the clutch
is perfect for the rest of the drive and remains good when I park the car.
This will repeat itself on the drive home after the car warms up on the
highway drive. Also - this motion is lost when the clutch is not used at all
during the highway drive - this morning, I was halfway into the highway
driving so I tested the free motion (without engaging the clutch) and sure
enough, it was there shortly after the engine temperature was up to 180F.
But once I did my first down-shift, going almost to the floor, pedal is now
back to normal. There is no hydraulic fluid loss. Yesterday, I tried
reseating the clutch line fitting on the back of the master cylinder since
it looked wet but that had no effect. Still, it appears to be air getting
pulled into the system, with possibly air expanding (the thermal component),
or at least that is one theory of mine.

Anyway, I am at a bit of a loss. Any tips on further troubleshooting? If
nothing else, I'll do a disassembly of the whole clutch hydraulics this
winter (slave, master, and hose).

David Councill

64 MGB

67 MGBGT

72 MGB (in process of re-installing OD transmission)

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vlink=3Dpurple><div class=3DWordSection1><p class=3DMsoNormal>I&#8217;ll =
ask the question before I start any serious disassembly. My near daily =
driver, 1964 MGB, is still on the road although the next few months I =
will likely drive it a lot less as darkness descends and roads start =
getting icy. A month ago, I had seal failure on my clutch master =
cylinder, resulting in the puddle of hydraulic fluid near my feet. Upon =
disassembly, I saw some pitting in the bore which caused premature seal =
failure (I rebuilt this unit about 5-6 years ago, not sure if I missed =
the pitting). So I decided to replace the master cylinder with a new =
Lockheed unit. Now I have a new problem &#8211; pedal motion is perfect =
until the car warms up and then most of the pedal resistance is lost =
briefly. Typically, when I drive to work (or leave work), the car starts =
out cold, warming up on the short city drive, and then I have 8-12 miles =
of highway driving. The first time I use the clutch after the highway =
drive, the pedal goes almost to the floor with no resistance but enough =
to still work. But once I do that one time, the clutch is perfect for =
the rest of the drive and remains good when I park the car. This will =
repeat itself on the drive home after the car warms up on the highway =
drive. Also &#8211; this motion is lost when the clutch is not used at =
all during the highway drive - this morning, I was halfway into the =
highway driving so I tested the free motion (without engaging the =
clutch) and sure enough, it was there shortly after the engine =
temperature was up to 180F. But once I did my first down-shift, going =
almost to the floor, pedal is now back to normal. There is no hydraulic =
fluid loss. Yesterday, I tried reseating the clutch line fitting on the =
back of the master cylinder since it looked wet but that had no effect. =
Still, it appears to be air getting pulled into the system, with =
possibly air expanding (the thermal component), or at least that is one =
theory of mine.<o:p></o:p></p><p =
class=3DMsoNormal><o:p>&nbsp;</o:p></p><p class=3DMsoNormal>Anyway, I am =
at a bit of a loss. Any tips on further troubleshooting? If nothing =
else, I&#8217;ll do a disassembly of the whole clutch hydraulics this =
winter (slave, master, and hose).<o:p></o:p></p><p =
class=3DMsoNormal><o:p>&nbsp;</o:p></p><p class=3DMsoNormal>David =
Councill<o:p></o:p></p><p class=3DMsoNormal>64 MGB<o:p></o:p></p><p =
class=3DMsoNormal>67 MGBGT<o:p></o:p></p><p class=3DMsoNormal>72 MGB (in =
process of re-installing OD transmission)<o:p></o:p></p><p =
class=3DMsoNormal><o:p>&nbsp;</o:p></p><p =
class=3DMsoNormal><o:p>&nbsp;</o:p></p></div></body></html>
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From: PaulHunt73 via Mgs <[email-masked]>
Date: Tue, 8 Nov 2016 08:52:21 -0000

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"pedal goes almost to the floor with no resistance but enough to still =
work"

Expand?  If the biting point is very low when this happens, then either =
the pressure seal is not sealing when it should be, or there is air in =
the system.  If the biting point remains 'normal' then it's a purely =
mechanical issue, possibly pivots binding with heat expansion.

If the banjo union at the master is wet then that does imply a leak, =
which may or may not let in air, and further tightening may not seal it. =
 Also if it's wet it implies fluid loss, which should eventually become =
visible from the level in the master.  But that may take some time, and =
it will be affected by fluid expansion and contraction with heat/cool =
cycles.

If there is air getting in, then I can see that this becomes apparent on =
flooring the pedal, which will tend to push the air down while =
compressing it.  But it will float to the top of the pipe while the =
pedal is down, then when the pedal is released it will be flushed out of =
the system into the reservoir.  That is one of the many techniques that =
may be needed to get full travel of the clutch slave after working on =
the hydraulics.  The effects of incomplete bleeding i.e. air remaining =
in the system would normally be apparent on every pedal operation.

However that begs the question of how the air is getting in in the first =
place.  The fluid is either under no pressure at all (pedal released), =
or under pressure (anywhere beyond where the master piston seal closes =
off the bypass hole to the reservoir).  If air was getting with the =
pedal released then it would be apparent on the first operation no =
matter what the temperature was, and more than once on each drive.  And =
the pressure while the pedal was anywhere in its disengagement portion =
should stop air getting in, even on the upwards stroke, as it is the =
cover plate that is pushing back on the slave piston and hence the =
fluid, which is the main factor in returning the master piston and hence =
the pedal.

The other possibility is that something is causing the slave piston to =
be pushed back into the cylinder while you are driving, pushing fluid =
that should normally be in the system back into the reservoir, and =
creating lost-motion at the release arm.  This has to be taken up before =
you can start to disengage the clutch, so reducing pedal back-pressure =
and lowering the biting point.

But I can't really see how that could suddenly start happening with the =
new master, so the new master (subject to getting a proper seal on the =
banjo) has to be the prime suspect.

PaulH.

----- Original Message -----=20
  ... Now I have a new problem - pedal motion is perfect until the car =
warms up and then most of the pedal resistance is lost briefly...
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<DIV><FONT size=3D2>"</FONT><FONT size=3D3>pedal goes almost to the =
floor with no=20
resistance but enough to still work"</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT size=3D2>Expand?&nbsp; If the biting point&nbsp;is very low =
when this=20
happens, then either the pressure seal is not sealing when it should be, =
or=20
there is air in the system.&nbsp; If the biting point remains 'normal' =
then it's=20
a purely mechanical issue, possibly pivots binding with heat=20
expansion.</FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2>If the banjo union at the master is wet then that =
does imply a=20
leak, which may or may not let in air, and further tightening may not =
seal=20
it.&nbsp; Also if it's wet it implies fluid loss, which should =
eventually become=20
visible from the level in the master.&nbsp; But that may take&nbsp;some =
time,=20
and it will be affected by fluid expansion and contraction with =
heat/cool=20
cycles.</FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2>If there is air getting in, then I can see that this =
becomes=20
apparent on flooring the pedal, which will tend to push the air down =
while=20
compressing it.&nbsp; But it will float to the top of the pipe while the =
pedal=20
is down, then when the pedal is released it will be flushed out of the =
system=20
into the reservoir.&nbsp; That is one of the many techniques that may be =
needed=20
to get full travel of the clutch slave after working on the=20
hydraulics.&nbsp;&nbsp;The effects&nbsp;of incomplete bleeding i.e. air=20
remaining in the system would normally be apparent on every pedal=20
operation.</FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2>However that begs the question of how the air is =
getting in in=20
the first place.&nbsp; The fluid is either under no pressure at all =
(pedal=20
released), or under pressure (anywhere beyond where the master piston =
seal=20
closes off the bypass hole to the reservoir).&nbsp; If air was getting =
with the=20
pedal released then it would be apparent on the first operation no =
matter what=20
the temperature was, and more than once on each drive.&nbsp; And the =
pressure=20
while the pedal was anywhere in its disengagement portion should stop =
air=20
getting in, even on the upwards stroke, as it is the cover plate that is =
pushing=20
back on the slave piston and hence the fluid, which is the main factor =
in=20
returning the master piston and hence the pedal.</FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2>The other possibility is that something is causing =
the slave=20
piston to be pushed back into the cylinder while you are driving, =
pushing fluid=20
that should normally be in the system back into the reservoir, and =
creating=20
lost-motion at the release arm.&nbsp; This has to be taken up before you =
can=20
start to disengage the clutch, so reducing pedal back-pressure and =
lowering the=20
biting point.</FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2>But I can't really see how that could suddenly start =
happening=20
with the new master, so the new master (subject to getting a proper seal =
on the=20
banjo) has to be the prime suspect.</FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2>PaulH.</FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</DIV>
<DIV>----- Original Message ----- </DIV>
<BLOCKQUOTE=20
style=3D"BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; =
PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
  <DIV style=3D"FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: =
black">... Now=20
  I have a new problem =96 pedal motion is perfect until the car warms =
up and then=20
  most of the pedal resistance is lost =
briefly...</DIV></BLOCKQUOTE></BODY></HTML>

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