[Mgs] [MG-MGB] Clutch problems with thermal component

David Councill dcouncill at karamursel.org
Wed Nov 9 08:54:42 MST 2016

I am still driving, squeezing out the final good weather days before winter.
Hopefully, I will get through today and tomorrow before I get a chance for
some disassembly. What I will do is drain the hydraulic fluid, redo the
banjo fitting connection, and remove/rebuild the slave cylinder. Then if the
problem persists, I will remove and investigate the master cylinder. Until
this car (1964 MGB), I have always rebuilt the hydraulics (seals) rather
than buy new master/slave cylinders. But now I have the new Lockheed master
cylinder. The slave cylinder was a new aftermarket unit I installed 5-6
years ago when I started the restoration. The clutch components (pressure
plate, clutch disc, throw-out bearing) were installed new when I rebuilt the
engine last year, about 5000 miles ago.


As near as I can tell, this is a one time thing - the free play starts to
increase about two miles into a 60 mph cruise and when the coolant
temperature approaches normal operating temperature (185 F). Most of the
time, the drive is as I described - very brief city driving, 12 miles on the
highway, and then into the next city for a couple of miles to work. I will
get about 80% free travel on the clutch during the highway drive -
depressing it restores it to maybe 40% travel after the first push and after
2-3 more pushes (standard shifting), I will be back to normal (maybe 5% free
travel). Fluid levels are unaffected. Sometimes, I take an alternate route
home which is almost twice as long but has two highway drives and goes like
this - 1 mile city, 8 miles highway, 2 mile city, 17 miles highway, home.
That is usually my drive to get cheaper petrol so I do it about once a week
(more frequently during convertible season just for the extended drive). And
the problem then also occurs in the initial warmup highway drive, not on the
second highway leg but then that was only one trip.


Barring any unforeseen circumstances, I'll have an answer in a few days.
Hopefully, as I am short of backup cars. My 72B is partially disassembled -
just about ready to pull the engine/transmission in order to put my OD
transmission back in as well as fix the rear engine seal (plus install
speedi-sleeve). The 67 BGT has been idle for about four months due to
excessive oil consumption (~80-100 miles to the quart) and its up next for a
rebuild when I get done with the 72. 


David Councill

64 MGB

72 MGB



From: PaulHunt73 [mailto:paulhunt73 at virginmedia.com] 
Sent: Wednesday, November 09, 2016 12:28 AM
To: David Councill; MG-MGB at yahoogroups.com
Cc: mgs at autox.team.net
Subject: Re: [Mgs] [MG-MGB] Clutch problems with thermal component


Firstly the tilt in the later clutch master - that was to clear the larger
brake reservoir cap on North American models when they gained the dual
circuit system, but was fitted to all MGBs.  AFAIK the tilt rotates the
master cylinder about the centre-line of the bore, so everything should
remain in line.  Whilst the pipe could be moved to mate up with the master
outlet, if the position of the bore changed relative to the pedal that could
cause problems.


1.  If you continue highway driving, after you have experienced the long
travel and corrected it with a couple of pedal presses, does it happen
again?  Or does it only happen once no matter how far you drive?  If it only
ever happens once then it looks like a master seal problem.


2.  Another test, once you feel the slackness, stop the car where safe
without using the clutch, and see if you have any free play in the release
arm.  If there is, then something would appear to be nudging the piston back
into the bore.  However, there should be a spring inside the cylinder which
is constantly trying to push the piston out, and that should continually
being taking up any free play.  The purpose of this is to take up any and
all play from there on i.e. in the release bearing and friction plate, it is
this that makes the MGB clutch self-adjusting for wear.


As another part of this test, at various temperatures, by pushing on the
release arm move the push-rod and hence the piston all the way back into the
cylinder, let it go, and it should ease its way out again.  Make sure the
master reservoir has some space at the top to avoid the fluid overflowing.




----- Original Message ----- 


It is a bizarre problem. I was hoping someone might have had similar
problems. Since my last email, I have made two 15 mile drives with the same
very predictable issue. The master remains the primary suspect because the
issue happened after replacing the master cylinder. The slave cylinder was
an aftermarket replacement I put in about 4-5 years ago but the only thing I
have done with it is to drain the brake fluid.


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