Master Cylinder Reseal

Recently Sheila the Triumph decided that she was going to eject all her clutch fluid into the driver's footwell thanks to a blown seal in the clutch master cylinder.

Luckily, seal kits for these are fairly easily available and cost a whole lot less than a new cylinder.  Fitting them isn't too hard, and the job is very similar to resealing the typical Girling brake master cylinders used on these cars so this is a sort of dual-purpose how-to.

Note that with a brake cylinder, especially, everything really must be spotless as it goes back together and if there is any doubt at all about the condition of the cylinder bore then don't repair, replace! If a clutch cylinder fails again because of dirt damaging the seals it's annoying but the consequences of sudden brake failure are likely to be far more serious than a bad mood!

With that little warning out the way, let's get on with the job.

The cylinder is connected to the pedal by a pushrod which is held in place by a clevis pin:

Clevis pin attaching pushrod to pedal


This is removed simply by pulling out the split-pin that's fitted to the right hand (in the photo) end and pulling it out.  There is a washer under the split-pin that you should try not to lose!

Pushrod disconnected from pedal


Next disconnect the fluid pipe, or pipes, from the cylinder body.  These cars originally had a master cylinder with an integral fluid resevoir but a previous owner has replaced Sheila's with one using a remote resevoir.  With the original cylinder there is only one pipe to disconnect.  I also found that the pipe for the resevoir was stuck pretty solidly in the cylinder so, rather than risk damage to the union, I disconnected where the metal and flexible pipes had been joined further up:

Disconnecting the fluid pipes

Fluid pipes disconnected


The cylinder is now only held by two nuts which can be undone, allowing the cylinder to come free:

Undoing cylinder nuts

Removing the cylinder


This is the point where, on a brake system, cleanliness should start.  Being a clutch system, and a repair rather than restoration, I gave the cylinder a quick wipe down to remove loose dirt and proceeded to strip it.  On a brake one, or for a "pretty" job, now is the time to clean the outside of the cylinder thoroughly.  The old seals can serve one last useful purpose by keeping any dirt, grit and solvents out of the bore while you do this.

Once you're ready to strip the cylinder, pull off the old dust boot from the pushrod and remove the circlip that holds it in the cylinder:

Remove pushrod circlip

Pushrod removed


Assuming the cylinder isn't siezed (in which case you should be replacing it) it should now slide out of the cylinder.  Gently tapping the cylinder downwards onto a wood block should start it moving, allowing you to pull it out:

Removing the piston

Piston removed from the cylinder


The next stage is to clean and examine the bore of the cylinder.  there's no point trying to reseal a cylinder with a damaged bore.  A repeat failure of a clutch will be a waste of time and continuing to use a worn or damaged brake cylinder could literally be fatal!

Use soft cloth and clean brake fluid to carefully clean the whole length of the bore, remembering to allow the fluid to flow through the inlet (if applicable) and outlet connections.  Then wipe the bore with soft rag to dry:

Cleaning the bore


Inspect the bore for any wear - this will tend to show as shiny patches on the surface - or scoring.  very slight scoring on a clutch cylinder might be worth going ahead with resealing at this point but any wear or damage in a brake cylinder means replacement is the only safe option.

Assuming that all's well with the bore, you can then move on to fitting the new seals.  The stripped piston / valve assembly and contents of a seal kit look like  this:

Stripped piston and seal kit


The piston is in two parts. On the left of the photo above is the valve assembly, which allows fluid into the cylinder from the resevoir when needed, and to the right is the main seal that forces fluid to the clutch.  They're joined together by the cylindrical clip inside the spring.  To separate them, gently lift the tag on the clip and the two parts will come apart:

Unclipping valve retainer

Valve and piston separated


At this point the main seal can be simply slipped off the piston, the piston cleaned, and the new seal slipped carefully on, making sure it's the right way round:

Main seal removed from piston


The valve rod is held in the retainer by a key-hole slot.  Unhook the rod and remove the spring, the valve cage, and the wavy washer:

Valve stripped


This allows you to reach the valve seal which is simply slipped off the valve body and, after cleaning, replaced with new.  Again, make sure you're fitting the new seal the correct way round!

Reassemble the piston, making sure that the valve retainer has clipped securely back onto the end of the piston:

Piston reassembled with new seals


Once the piston is reassembled, lubricate the main seal and piston body with the rubber grease and ease back into the cylinder:

Grease piston before reassembly

Refitting greased piston


Also grease the end of the pushrod before fitting.  This will help to avoid the possibility of damp getting in and corroding the cylinder:

Grease the end of the pushrod before fitting


Replace the circlip and ease the new boot over the top of the pushrod fork.  Two thin screwdrivers and some more rubber grease help here:

Refitting the pushrod boot


You now have a cylider ready to refit which, as guides like this tend to say, is the reverse of the above:

Cylinder ready for refitting


One final job once it's all together is to bleed the system. The bleed screw for this car is reasonably accessible from under the bonnet but other models may not be so well designed:

Bleed screw with pipe attached


I like to use a "one-man" bleed tube which can be bought cheaply or made yourself.  It's simply a length of flexible tube (blue silicone fishtank airline in the photo) with one end blocked (a screw does fine for this) and a slit, (about 10mm long) just above the screw.  Fluid can be forced out through the slit under pressure but air won't get sucked back through it.  Simple, effective and reasonably quick to use.  It also avoids the need to find someone to pump pedals for you!

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