Clutch master cylinder rebuild - bad parts or bad bleeding method?

[Roach15]

Hi guys,

I had a large service done recently here in South Africa which included replacing the clutch fluid. When I got the car back, the clutch started randomly engaging at different pedal locations and gradually getting worse. Eventually reversing into my garage became a huge PITA. There was no leaking fluid so a little research showed that it was probably the master cylinder.

I got a rebuild kit from a store here and got to work on the MC (first time). The seal was ripped. When replacing the cylinder and trying unsuccessfully to replace the c-clip I realised that the c-clip was different size to the original. I then had a look at the whole setup and realised that the dimensions were ever so slightly different for the piston length (approx 1mm) as well as the seal diameter (approx 0.5mm). The main piston diameter is exactly the same width.

Now all the stores are closed and I can't get another kit, but I don't see why the kit isn't working anyway. I keep ending up with the clutch 'popping' at 3/4 which I assume is the seal leaking?

I've bled the system a few different ways. Firstly with the MC attached, a ziplock bag around the MC and using my finger to block the MC outlet to main clutch lines. Then I bench-bled it a similar way, covering my workbench (and me) with fluid and then reinstalled. I've swapped the new seal on the old piston, tried a whole bunch of stuff and always end up at the same point. The clutch is 'popping' at 3/4 and I can't get pressure to bleed at the slave.

I thought I had gotten on top of this early but now it's last minute and we have a 2000km trip planned for Tuesday. I'm trying to figure out if I'm just not bleeding everything properly or am I potentially using a faulty part? Or is there possibly something else wrong - slave cylinder?

This forum has saved my skin in the past - I'm hoping that it will again! Thanks a lot guys.

 

[Malleus]

If you don't have the correct rebuild kit, it's likely that you won't get a good seal in the master cylinder bore. If the pressure doesn't build, the slave won't move, or won't move enough.

Your original problem sounds like a linkage related problem. I'd check the pivot pin near the slave cylinder. I wouldn't be surprised to find that either the pin is undersized or the bore has been opened by a wobbly pin.

 

[Roach15]

Thanks Malleus! I've isolated the problem to the master cylinder and repeated the failing seal by hand on the bench. It takes a fair bit of pressure to break the seal initially and then after that it doesn't hold any pressure. When I take it apart again there is no visible damage on the seal.

It's still kinda strange to me that the seal is failing just because it is slightly too large. I would have thought that if it fits snugly without any creases then it should hold pressure. In any case the cylinder in the rebuild kit is the correct diameter.

I've taken the slave apart and it seems to be fine. How would an undersized pin cause an intermittent problem? And wouldn't an opened bore then show fluid leakage?

Hoping that I can find a spare parts store open today, but I don't like my chances - this whole city shuts down at this time of year.

BTW i've now got all the proper bleeding kit so I've been bleeding the master on the bench into a tube to a raised container, and siphoning fluid through the main brake lines for about 100mls so I'm sure now that I'm not leaving any holes anywhere in the lines.

 

[Malleus]

The oversized hole (we call it "wallowed out" around here) would tend to develop an egg-shaped profile. This allows the pin, and linkage, to function at the correct distance at times, and at others to be either too far away or too close.

The clutch hydraulic system is the easiest to bleed. The line is short and only has two entry/exit points. If you can't bleed it on the first, or worst the second try, the fault is in the cylinder seals, at one end or the other. Typically the fault is at the master cylinder.

The seals have to be exactly the correct diameter; too large or too small won't work. They're designed to distort under pressure, so they should be just undersized (you may not be able to "see" this) so as to fill the gap under pressure. It's also fairly important that the bore is "clean"; no burrs, scratches or other imperfections.
HTH