1981 GS850G Front Master Cylinder

[danmcp]

Hey guys!

In the process of restoring a 1981 GS850G and it came with a shot front master cylinder and after trying to rebuild and the brittleness of the reservoir, the best thing for me to do is just replace the whole thing - which is proving a task. The MC it comes with has a 14mm bore but we're looking at dual disc brakes so after a lot of research and sifting through different posts, I'm still at a loss as to whether I can replace with just another 14mm MC and it's existing hoses or if that'll be dangerous.

Any advice/links to some replacements would be awesome!

Thanks in advance

 

[ged]

Hi danmcp, I was going to replace my front master cylinder. But in the end I have decided to just purchase an OEM rebuild kit for the master cylinder will be much better than an aftermarket one. Regards

 

[danmcp]

Hi danmcp, I was going to replace my front master cylinder. But in the end I have decided to just purchase an OEM rebuild kit for the master cylinder will be much better than an aftermarket one. Regards

Hi Ged,

Thanks for the tip but the plastic reservoir itself is extremely brittle (nearly 40 years will do that to ya) and I did get an OEM rebuild kit to try and do the deed on it but the piston in the kit was too big for the bore so I'm inclined to think that the 14mm cylinder isn't original anyway - which is why I'm thinking of replacing the whole thing altogether. I reckon I got a 5/8 piston in the rebuild kit which is designed for a 5/8 bore built for the dual discs - but could just be guessing as well. This is where I'm hoping for the best guidance - will a 14mm be enough juice for the dual calipers or should I look at an aftermarket 5/8 cylinder for added safety/braking juice?

 

[Nessism]

5/8 is the stock size for dual disc bikes. 14mm is for single disc bikes.

 

[bwringer]

LOL, I'm looking at the exact same thing on a project GS850G; some numpty replaced the original 5/8"/16mm master cylinder with a 14mm version from some unknown single disc bike. And probably wondered why the brakes felt strange...

Anyhoo, the inexpensive replacement Chinese master cylinders from scAmazon and fleaBay can be surprisingly high quality, and I've used them a few times. Look for an item with a history of good reviews and ratings. You might also have better luck looking for 16mm master cylinders instead of 5/8". (5/8" = 15.875mm). The one I used came with an extra lever and a rebuild kit, and the switch was an actual rational design. It was pretty easy to make a little harness to plug it in.

Pay attention to the thread pitch on the banjo bolt and make sure you have or get a banjo bolt that will work; most aftermarket and other OEM brake components use 10mm x 1.25 banjo bolt threads, but Suzuki for some weird reason used M10x1.0 banjo bolt threads.

You'll also want to make sure you pay attention to the placement of the outlet to make sure it'll connect to your brake lines correctly; facing forward or in line with the bars.

The ergonomics can vary a little; I used one of these on my GS850G for a few months a while back because the rebuild kits from Suzuki were backordered. It worked fine, but didn't feel exactly the same, which sort of bugged me. When I finally got the kit from Suzuki I finished the rebuild and reinstalled my original. The difference was pretty minor, and unless you've already ridden the bike for many years and miles you'd never notice.

 

[Guest]

BWRinger that is very good info. I was about to post my success with using a 16 mm aftermarket MC on my 78 Honda Gl1000 (dual front brakes) vs a 14 mm which did not have enough oomph. I will file this away, you have explained the thread pitch difference on the Suzuki and likely have save me a fair bit of head scratching as it looks like I will be replacing the front MC on my GS850. Thanks and sorry to butt in danmcp

 

[Grimly]

I used an XJ900, then a NT650V (Deauville) m/c on mine over the years, both 5/8". The XJ was better than the original, but the NT was better again. Nominally all were the same hydraulic leverage but the better ergonomics over time made a difference.
I wouldn't have fitted a 14mm as the calipers are too prone to lost motion once they get a bit of wear on them, and you don't want that translating to lost lever motion at the bar end of the system.
On a system where the calipers don't have that lost motion with wear - ie, twin opposed cylinders, you can really benefit from the extra leverage given by a smaller m/c.

I tried a 14mm m/c on my XS and the brakes were fantastic, but changed to a 5/8" m/c (once again, a NT650V) and they're still bloody good. Much better than stock, as Yamaha fitted an 11/16" m/c for some weird reason.

 

[danmcp]

Amazing guys - thank you so much. Brilliant advice!

I will chase up a 5/8(16mm) MC and see how we crack on. Thanks again!

 

[salty_monk]

EX650 (Ninja) MC work great with stock or twinpot brakes. If you choose the right year you also get a compatible mirror mount. I've used pretty much all the different years at some point in my research...

 

[chris]

Hi, I used a m/c from a 06 sv650s with great results. Bonus is that the fancy folding/adjustable cnc levers you can buy on flea bay will also fit the clutch perch.

 

[tas850g]

Hi, I used a m/c from a 06 sv650s with great results. Bonus is that the fancy folding/adjustable cnc levers you can buy on flea bay will also fit the clutch perch.

I’ve got my ‘04 SV650N MC on my ‘79 850. I couldn’t ask for anything better.

A question for OP.....do you want a remote reservoir or one that is built into the MC? I used my N model SV MC because that MC has the reservoir built into the MC with the mirror mount. The S SV has a remote reservoir but no mirror mount.

 

[Big T]

Since no one else has said it, replace your brake hoses
They're 35 years past their replacement date

 

[salty_monk]

You could also add twinpots and bigger discs to that model if you wanted to... (Click the link below for the longest thread ever...! )