Master Cylinder rebuild - what goes where?

Rebuilding my master cylinder, but I don't know which rubber o-rings go where on the piston.

There are two, and one has a smaller inside diameter than the other.

Anyone remember from their rebuild?

All set with the rebuilt caliper and stainless lines and itching to get the season started now that the snow has melted 8)

 

Splazoid -
I do not know the answer but someone here will get to you soon.

Where in Wisconsin are you? I am getting sick - I put off doing my front and rear brakes, valve clearance check and front fork seals and NOW with lows in the 40's it sure is riding season!!!

Worse than that is that the Frozen Snot Ride is next weekend, won't make it this year,,, won't be too frozen either. . . . :wink:
I'm not going to rush through doing any of my repairs but right now I wish I would have started all of this earlier!
Maybe next week I'll stop being a procrastinator. :lol:

 

The rubber "cap" goes on the end of the spring. The small o ring goes on the piston (a real pita). The larger rubber ring is the dust cap on the outside of the MC.

There is a great walkthrough with pics on the forums. Sorry I don't have the link but just search master cylinder rebuild and scroll down til you find it.

 

PS use a little brake fluid to lube the rings as you assemble it.

 

Here you go

I recommend using more than a little brake fluid.

And a pair of rubber gloves.

 

I live about 30 minutes from Green Bay. I'm feeling the spring fever much like you are. Fortunately I've got pretty much all of the parts I need to finish my repairs. Depending on how far away you are, I'd be willing to give some pointers/trade shims etc.

Ride smart

 

Thanks for the help, but I've read that already - it fails to answer my question exactly.

I have a 550, so the master cylinder is a bit different. I do not have a "cap" that would go on the end of my spring, rather there are two differently sized o-rings which go on the piston (at least that's what was on the old one). The rebuild kit included a new poston shaft, 2 o-rings and a dust seal. I just cant figure out which o-ring goes in which of the two places on the shaft.

 

Technically, they're Hydraulic Seals; not O-rings.

The "Thick" Seal fits tightly at the outboard end.
Fits the locating channel on the Plunger -- "Tightly" -- and SEALS the Fluid from escaping out the Lever-end.

The "Narrower" of the two is the Front Hydraulic Seal.
It often has a Wave-washer along with it.
The narrower of the two Seals goes on the Inboard-end of the Plunger and has a bit of "Travel" or "End play"

When the Plunger is released the Seal moves to allow Brake Fluid to seep through the tiny replenishment orifices and beyond the one0way seal maintaining the MC's ability to apply pressure as the pad wear down.

 

Awesome, thanks Rick. So, for specificity's sake, the thick seal is on the right, and the thin on the left?

Which direction do they need to be facing?

 

Take a close look at your parts.

The spots on the plunger where the various rubber parts go are all different diameters, which correspond to the different inside diameters of the rubber parts. They can all only go in one place on the plunger.

The "open" sides of the seals face "IN" (the direction the plunger moves when you pull the lever.)

Be sure the bore of the M/C is nice and clean.

Soak the two seals in a small cup of clean brake fluid before installing them, they'll be more pliable. Use lots of clean brake fluid to lube everything as it goes back together.

 

Perfect, thanks for the details.

 

If the "Guts" of the master Cylinder you are about to rebuild looked funky; it would be a good idea to examine the MC Bore AFTER it has been cleaned with Disc Brake Cleaning Fluid.

After cleaning, visually inspect the entire Bore.
Use a Pen-light type device and look-over the surface of the Bore.
Look for Build-up or Pitting.
Any Build-up needs to be removed.
Heating it with a Pencil-Tip flame of a Butane Torch will soften it and allow it to be cleaned without scraping the Bore.

If the Bore is Pitted ... You'll have to "Hone" the Bore.
Roll a tight Cylinder of 800 Finishing Paper on a Wooden Dowel
Lube the Roll of Finishing Paper with WD-40
Stuff the Rolled-up Cylinder of 800 into the Bore so that --> the roll does not unwind when rotated.

Use a Variable Speed Drill and Rotate the Roll in and out ... at different speeds to refinish the Bore and (hopefully) eliminate the Pitting.