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1979 GS1000S Front Master Cylinder Rebuild

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

Guest
Hi there from a newbie,

I'm currently overhauling a 1979 front master cylinder (the round reservoir type). I've bought a genuine Suzuki kit for it.

Despite all attempts (none that involve a hammer at this stage!), I can't get the new piston and seal into the bore of the master cylinder - the larger diameter of the seal catches on the leading edge of the bore and stops it dead. I've tried everything I can think of. The Suzuki workshop manual is of no use.

This task must have been done thousands of times, and I'm sure someone here could provide some advice.

I tried to insert several photos, but I'm only allowed to post 1. This might be a slow exercise due to my lack of familiarity with the posting procedure.

Any and all help will be gratefully received.
 

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What fluid did you place on that cup that made it swell like that?
 
Are you using an aftermarket seal kit or OEM?
 
All,

It is a genuine Suzuki overhaul kit.
The seal is the same size now (in the photo) as when it was first fitted.
It has not seen any petroleum based products or parts cleaner.
The only fluid it has been in contact with is brake fluid, as per the warning in the workshop manual.
 
If you are sure that the kit is the correct part number for your specific bike model, try using your fingers to gently press the lip end of the seal into the m/cyl bore. You need to "help" the seal into the chamfered edge of the m/cyl. If that does not work, cut a piece of plastic from a soft drink bottle or similar plastic container, wrap it around the seal, and tighten a hose clamp around the outside of the plastic just enough to compress the seal so that it slips into the m/cyl bore. Use a little brake fluid to lubricate the seal and plastic.
 
Here's a random thought.....is there only one version of the "round reservoir" type of master cylinder? or could there be different specs (I'm thinking of internal bore diameter here) depending on single or twin disc model?

The master cylinder I'm restoring wasn't fitted to the bike when I bought it - it was in a bucket of old bits that came as part of the deal. It may have come from any other GS model from the round reservoir era.

In the meantime I'll confirm with the vendor that the overhaul kit is the correct one.
 
There are several versions of the early round reservoir master cylinders. The reservoirs and corresponding O-rings to seal them are different dimensions to begin with. (three different sizes that I know of so far)
Check the part numbers/dimensions for the internal kits and I am guessing that they are different as well.
 
I went looking on the master cylinder casting for a part number. There isn't one, but there is "5/8" cast into the underside of the unit. Guessing that this might be the bore diameter, I checked the OEM workshop manual, and Bingo!!, the nominal bore is listed as 0.625". This means I do have the correct master cylinder for the bike. Now to source the correct cup, or salvage the crusty old cup that was originally removed.
 
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