[Mgs] Clutch problems wuth thermal component

David Councill dcouncill at karamursel.org
Mon Nov 7 09:13:29 MST 2016

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


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



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