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

Barney Gaylord barneymg at mgaguru.com
Tue Nov 8 18:07:26 MST 2016

Clutch pedal goes to floor on first stroke after 
a long rest, but works correctly otherwise.  This 
implies that the clutch slave cylinder piston has 
retractred too far, requiring one pedal push to reset it to correst position.

The clutch slve cylinder should have a light 
force compression spring inside.  See here: 

If that spring is missing the slave cyinder may 
still work, but the piston might retract farther 
than intended during a period of dormancy (long 
drive without using the clutch).  You might want 
to open the slave cuinder to check on what's inside.

At 08:26 AM 11/8/2016 -0800, David Councill via Mgs wrote:
>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.
>A recap of this morning’s typical drive – solid 
>pedal through several gear shifts through town, 
>started highway drive. During this highway part, 
>I slightly pressed on the clutch pedal several 
>times on the drive until resistance was felt to 
>see what was happening After a couple of miles 
>and coolant temp up to 160F, some slackness and 
>as soon as I got to the next town where I work, 
>I had almost full travel to the floor. But the 
>one press of the pedal and it was back to almost 
>normal, next time I shifted, the pedal was solid 
>again and a routine drive to work through town. 
>It should now be fine until I start the next highway drive.
>(if I was only doing city driving, I wouldn’t be having any issues)
>The symptoms appear more to be like a seal issue 
>as one to two presses of the pedal resolves the 
>issue (until next time). However, I have seen 
>hydraulic leaks before which are slight enough 
>to allow air in/out but not leak fluid. The seal 
>on the banjo is still the place of primary 
>suspicion so I’ll probably drain the master and 
>then completely redo the connection to see if I 
>can get a better seal. There is a slight wetness 
>at that connection but not enough to make for 
>even a minuscule drop of fluid. Also relevant, I 
>installed a new Lockheed master cylinder as I 
>mentioned. This is part LM13105, “fits all years 
>1962-1980”. However, sources like Moss list 
>separate clutch master cylinders for 62-67 and 
>68-80. From what I can tell, they are identical 
>with one small difference in how the master 
>cylinder mounts to the pedal box (how the bolts 
>line up) such that this new master cylinder does 
>not sit up straight but has a slight tilt 
>clockwise (5-10 degrees). That could be enough 
>that the flare fitting might be damaged or 
>slightly off on the banjo connection – or at 
>least that is my primary theory at present. The 
>only problem with that theory is that somehow 
>air would be getting pulled in under no pressure 
>or vacuum other than that generated by metal 
>expansion. Hopefully, it is not the master 
>cylinder and its seals at fault because that involves a lot more work.
>Still, I thought about asking the group about 
>master cylinders before I started – re-sleeve 
>versus new. The costs were about the same so I 
>went with new. Maybe I should have tried one of 
>those new plastic reservoir after-market units 
>since this is my almost daily driver. For now, I 
>will work with the banjo fitting but I may end 
>up trying another master cylinder if that fails, 
>perhaps take the one out of my 67 BGT which is 
>slated for an engine overhaul due to worn rings 
>and excessive oil consumption. Either way, I’ll 
>get this figured out sometime in the not too 
>distant future and then I’ll report back.
>David Councill
>64 MGB
>72 MGB

>From: Mgs [mailto:mgs-bounces at autox.team.net] On Behalf Of PaulHunt73 via Mgs
>Sent: Tuesday, November 08, 2016 12:52 AM
>To: MG-MGB at yahoogroups.com; mgs at autox.team.net
>Cc: MG-MGB at yahoogroups.com
>Subject: Re: [Mgs] [MG-MGB] Clutch problems wuth thermal component
>"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.
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