1981 GS850G rear brake master cylinder (I need help)

[Slowride]

Original piston and spring on the left, new rebuild parts on the right.

image.jpg

[Slowride]

Please read my previous three posts. Where does the "cup" (rubber seal) fit in with my rebuild? I have looked at diagrams but they are not clear. In the diagrams, it looks like it might fit on top of the piston and the base of the spring fits inside of it. When I took my master cylinder apart, there was not a rubber cup in it. Maybe I'm assembling it wrong. I thought when I took apart my master cylinder, the fatter part of the spring came out first. Does the tapered end of the spring go into the cylinder first or is it just the opposite?

[bwringer]

OEM is always, always, always, always best if it's available. And in this case, it is available. K&L parts are very often problematic, and their application charts are often off base.

That said, these imitation parts are what you have in hand and they might even work. Just be aware that they also might not.

Hard to tell -- the piston and seal are definitely visibly different, but I don't know whether they differ in a way that matters.

Here's the OEM parts diagram:

https://www.onlinecycleparts.com/oemparts/a/suz/50d40183f8700230d8b4e4f8/rear-master-cylinder

In your photo, no one can really tell for sure what that added blob is.

It may be a boot, #6 in the diagram.

It may also be a different seal, and you're intended to change it depending on the application -- note how the seal in the imitation part seems shorter than the seal in the original. The blob seems like it might possibly be a taller version of the seal? Hard to tell.

This is a better-lit photo from K&L's website showing a completely different master cylinder kit. The thingy on the left is the boot that keeps muck out of the MC. The blob in your photo is differently proportioned, but if it looks like it performs a vaguely similar function, then that might explain it too.

[Slowride]

So, does the tapered part of the spring go in first or does the larger diameter end go in first? I put the wider end in first, then the smaller diameter end seemed to fit into "cup" with the bottom of the cup coming in contact with the piston. It seems to move ok but I'll have to see what happens when I put it back onto the bike and connect the brake line. I may just end up buying the OEM parts.

[Slowride]

Oh, by the way, years ago when I was working on my bike you (bwringer) helped me a lot. Thanks, I appreciate your time and expertise.

[bwringer]

Hmmm... I have no memory of which way that spring goes in.

And I don't feel like taking mine apart to check...

Someone? Anyone?

[Slowride]

Installed the master cylinder, bled the brakes, took it for a ride. It seemed OK (yes, just OK) but I think I'll take the calipers apart and check the pads. Any idea how often the pads should be changed? Although I don't have many miles on the bike, 14,300 (I'm ashamed) I'm guessing the pads should be changed since they are the original ones.

[bwringer]

You change the pads when they're worn out. Or if they get contaminated with oil for some reason; front pads, for example, must be replaced if they get fork oil on them.

If yours look good, then you can keep using them. However, there's also an argument for replacing them with modern pads so you can benefit from 40 years of advancements in brake pad compounds. This matters a lot more on front brakes, but the improvement from a fresh set of EBC red or black pads is downright astonishing compared to the 40 year old factory set.

[2BRacing]

So, does the tapered part of the spring go in first or does the larger diameter end go in first? I put the wider end in first, then the smaller diameter end seemed to fit into "cup" with the bottom of the cup coming in contact with the piston. It seems to move ok but I'll have to see what happens when I put it back onto the bike and connect the brake line. I may just end up buying the OEM parts.

Yes, the larger diameter end of the spring with the larger metal "cup" goes in first. That rubber cup that was at the right hand side of your photos of the repair kit in posts #15 and #16 goes between the smaller diameter end of the spring and the flat side of the piston as per the photo below. Looks like the rubber cup between the piston and the spring was missing in your original assembly (photo in post #14 and left hand side of post #16) - that was the cause of no pressure build-up and no fluid coming out!

[Slowride]

Thanks. I think I'll take your advise and get some new pads. It can't hurt.

[Slowride]

Thank you 2BRacing. That is how I ended up assembling it, I guess I got lucky. Yes, when I originally took the master cylinder apart, there was not a cup in there, just the piston and spring. I thought I remembered which direction the spring came out but, I forgot. (I'm blaming that on my age) Would you happen to know, when replacing the rear brake pads, if they are designed for placement in the calipers dependent on the direction the rear wheel turns?

[2BRacing]

.......when replacing the rear brake pads, if they are designed for placement in the calipers dependent on the direction the rear wheel turns?

If new pads are put in, direction will not matter. If you are putting used pads back, they may have worn according to the surface of the disc.

Place a piece of sandpaper on a smooth, flat surface (like a piece of glass) and sand away grooves in the pad surface.

[Slowride]

Thank you 2BRacing. Hopefully, I'll get to the calipers and pads today as long as my wife doesn't have too long of a list for me, of things to do.

[Slowride]

Thank you salty_monk for the tip on using air pressure to get the pistons out of the rear calipers. Took the calipers apart (yesterday) cleaned out all the gunk and sanded the pads, which were not too bad but I think I'll still order some new ones. To be safe, I used my little compressor (to get the Pistons out) outside on the lawn and the first piston only went about 3 feet, barely. So I figured I needed more pressure and sure enough, the other one went flying. If my neighbors were watching, I'm sure they got a good laugh. Anyway, put it all back together, put it back on the bike, bled the lines and went for a ride. Brakes work great (again).

Thank you all of you who gave me great info and tips.