Need to replace front master cylinder

[Frank]

I need a new master cylinder, I am confident the one on my 1981 gs750e is not the original, it leaks, but I am totally confused what to buy. It has dual disc but every cylinder I find saying its for this bike has a 14mm piston bore. The 5/8 ones I find on eBay say they will not fit this bike. The one on there now is 5/8 but I know it's not original. Do the 5/8 cylinders for dual disc brakes have two lines of the cylinder? Is this my mistake and do I need the 14mm. Or do I need the 5/8 regardless of how many lines come out of cylinder? ( there is only one going into a conjunction under head light, then two seperate lines go to each disc). Can someone recommend a master cylinder for this bike, one with a mirror tap as the one there now has none so I have no mirror on that side. Thanks soo much in advance. Sorry to be so clueless first street bike at 49 trying to make the transition from riding in the dirt.

 

[tom203]

P/N 59600-45130 used on......
Alpha-Sports


- Part Number Cross-Reference - Where Used


Part Number or Accessory Name


1980 SUZUKI GS1000GT - FRONT MASTER CYLINDER
1980 SUZUKI GS750ET - FRONT MASTER CYLINDER
1980 SUZUKI GS850GT - FRONT MASTER CYLINDER
1981 SUZUKI GS1000GX - FRONT MASTER CYLINDER
1981 SUZUKI GS750EX - FRONT MASTER CYLINDER
1981 SUZUKI GS850GX - FRONT MASTER CYLINDER

new one can be had for about $200 ( try parts outlaw ) look at this parts fiche to see how mirror works

http://www.alpha-sports.com/spst/1981 GS750E/22.htm?q=suzuki-parts

 

[ShadowFocus603]

You need the 5/8 or 16mm master. Ordinarily there is either a single line off the master that gets split on the bottom side of the forks or there are two lines start to finish. Either way you have to have the larger master or you will not have the same stopping ability as stock. There are several Chinese masters that can be had cheap on eBay and the like that are decent quality parts. There are some that opine against anything Chinese but the fact of the matter is a Master cylinder is a stone simple piece of equipment and nye on impossible to get wrong.

http://www.ebay.com/itm/5-8-16mm-Br...Parts_Accessories&hash=item417c1319a1&vxp=mtr

 

[Frank]

Ok going to buy this one of eBay, entered bike make, year ect and it says it's not compatible. Should I disregard this and buy it anyway?

 

[ShadowFocus603]

Your bike has 7\8 bars and dual discs. It will work. I've used that same model in the past. Never had a problem. It even has the lug for the mirror to screw into.

 

[Frank]

Ordered it thanks

 

[ShadowFocus603]

When you get it, the best way to bleed it is by forcing fluid back up to it from each caliper using either a capable bleeder or a syringe with hose attached to it. Then, zip tie the brake lever back to the grip for a day or two to allow the micro air bubbles to fully leave the line and you'll be rewarded with firm solid front brakes.

This is the bleeder I got.
http://www.ebay.com/itm/1-man-brake..._Automotive_Tools&hash=item4191b6ef1d&vxp=mtr

 

[BADJACKSON]

I'm retracting this post. I did some more digging & I think I figured it out.

 

[tkent02]

You need the 5/8 or 16mm master. ..... Either way you have to have the larger master or you will not have the same stopping ability as stock.

Not true. If you have enough brake fluid movement to make the pads contact the discs you get more pressure to the calipers and better braking for a given lever pressure with the smaller bore. It's a simple mechanical advantage thing. I have put dual discs on several bikes and kept the same master cylinder, it works very well on each one.

If you have old stretchy rubber brake lines, sloppy calipers or have some air in the lines the bigger bore may work better, but if you have any of these things a better plan would be to fix it.

 

[8ball]

Not true. If you have enough brake fluid movement to make the pads contact the discs you get more pressure to the calipers and better braking for a given lever pressure with the smaller bore. It's a simple mechanical advantage thing. I have put dual discs on several bikes and kept the same master cylinder, it works very well on each one.

If you have old stretchy rubber brake lines, sloppy calipers or have some air in the lines the bigger bore may work better, but if you have any of these things a better plan would be to fix it.

This. Attached is a simple illustration, why:
The block on top represents the force applied on the lever.
The cylinder represents the Master Cylinder piston. (The left represents a piston size with a 1 inch square bore face, and the right one has twice the area)
The red Arrows represent the pressure exerted in the line

This diagram does not take into account the lever ratios of different master cylinders

 

[Grimly]

5/8th (16mm) m/cs are (I believe) the standard on those, but as said, if the normally sloppy calipers are still in place, you don't want to be losing caliper motion, losing brake lever motion, and gaining motion as you rear-end an SUV and flip your head over the roof of it.
Those are the perils of using too small a m/c on a system - if everything's in good shape with no lost motion, it can be fine, but there's something to be borne in mind - built-in reduncancy for a bit of wear and tear.
Rebuilt calipers, with good lines, (or better yet, something like Salty's conversion) would improve things a lot. A set of properly rebuilt later GS sliding-pin calipers are quite good, and the Kwack calipers that Salty's conversion depends on have a better design than Suzukis, so, as far as I can see the same problems of wear and tear don't present themselves; but a pair of opposed-piston ones are better. With opposed pistons of suitable size you can try out various bore sizes to see what suits you and not be worried about banging into the back of something. That's always assuming you can get twin-opposed piston calipers on yours, which I doubt you can.