Clutch M Cylinder failure below 55*F??

[Tony Hoffman]

It sounds like the master or slave cylinders are scratched inside.
I've certainly never had this problem, unless the hydraulics were bad.
Replaced, and no more problems. I've done the rebuilds on both the
slave and master, and I prefer this to buying "new" parts. That way, I
know what was done, and I've seen the bores myself. BTW, I've logged
at least 300K miles in 4000Q's, and currently have three serving as
DD's. So, I've done a few masters and slaves ;-)

As for the fluid, don't think it makes a huge difference. However, I
like flushing the complete brake and clutch system when I get a car,
and replace with synthetic DOT 4 fluid, usually Valvoline brand.

If you have any fluid inside the outer boot on the master or slave,
the seals are leaking, and something needs to be replaced.

I really can't imaging why the 55degree issue, as I've had them in
plenty of cold. I logged at least 2/3 of those miles in MT and WI.

Tony Hoffman


On Fri, Jan 9, 2009 at 10:04 PM, L DC <ldc007usa at yahoo.com> wrote:
> The clutch master cylinder in my '87 VW QSW (JT code engine/drive train same as 4KQ) fails when ambient temperature fall below 55*F.
>
> A bit of history regarding the matter since I have replaced the clutch master cylinder twice since mid 2006.
>
> In 2006, the clutch pedal in my '87 QSW began to sink sporadically on the way to my brother's house and by the time I got there, some 55 miles away, it completely collapsed.
>
> I replaced the clutch master cylinder and slave with "new" ones at a cost of about $130-$140 and $50-$60, respectively.
>
> About a year later, in a cooler-than-normal ambient temperature for South Fl or 60*F, the clutch pedal sunk to the bottom as I put the car in reverse to back out of the driveway.
>
> I immediately pulled the pedal with my foot and depressed it again to try to shift from reverse into first but nothing happened and car remained stuck in reverse.
>
> I shut engine off and tried pumping the clutch to see if I build hydraulic pressure to no avail.
>
> I hopped into another car and headed for work.
>
> Later that evening I get back in the QSW and everything worked as if nothing had happened, huh??.
> Ambient temperature had risen to normal levels, above 85*F.
>
> Over the next 3 or 4 months the car begins to experience slight clutch slippage sporadically when ambient temperatures are above 90*F combined with high humidity.
>
> Finally, last summer and some 300 miles away from home, the clutch pedal sank to the bottom, yet again on a cool morning (about 55*F) and never returned back up on its own even after the weather warmed up.
>
> That time I just replaced the seals within the clutch master cylinder at a cost of some $21 and the clutch worked nicely. The Sporadic clutch slippage experienced on hot days mysteriously disappeared as well.
>
> All was good until this week when the same symptoms began to manifest.
>
> This time, 50*F in early morning, again the clutch pedal sinks to the bottom and I'm not able to shift as there's no apparent pressure in the master cylinder.
>
> Once ambient temperature climbs to say 70*F or if I let the engine warm up some 15 minutes later, the clutch works fine.
>
> I can only imagine the problem being exacerbated up north where temperature this time a year hit below 0* !!
>
>
> Upon close inspection at the clutch master cylinder under the foot well, I noticed its accordion-like boot is wet with oil but not leaking.
>
> The system was completely bled following each clutch master cylinder replacement.
>
> So what gives??
>
> Are the seals on the master cylinder prone to failure below a certain temperature and or moisture build up inside the cylinder?
>
> Could the brake fluid be the culprit, even though I'm using the one specified one?
>
> Or could it be defective seals or scored clutch master cylinder wall?
>
> I read a little passage on the possible culprit from SJM Autotechnik website where it describes moisture buildup on the master cylinder:

> Any BTDT?
>
> Thanks for you valuable input as always.
>
> -Regards,
>
> Louis
>