Topic: Dual disc/master cylinder

[MauiK3]

As I inch closer to taking on a K3 restoration I was wondering if anyone has done the master cylinder to piston caliper area ratio on a dual disc conversion. I seem to recall that the last time I did this I used the existing single caliper master and it worked fine, just had a little more lever travel. If I read the vintage brake site correctly going up in size might actually make the brakes more wooden.
Thank you
Steve

[70CB750]

Yes, the original MC will work with dual disc setup.  I used it like that on the K0 with dual disc.

Also used early K MC on 78F front brakes, works great.

[CBR954AD]

Hey so on my 79 cb650 I did a dual disc conversion. I'm running a 79 gl1000 front end. However I cannot get my brake lever to be firm. I have all new components on brake calipers and brand new mc /lever
 However I believe it's for a single disc. I'm also using the gl1000 brake line splitter. No matter what I do my lever is soft. Brakes work but soft.
Do I need a master cylinder for dual disc?

[SOHC4 Cafe Racer Fan]

Hey so on my 79 cb650 I did a dual disc conversion. I'm running a 79 gl1000 front end. However I cannot get my brake lever to be firm. I have all new components on brake calipers and brand new mc /lever
 However I believe it's for a single disc. I'm also using the gl1000 brake line splitter. No matter what I do my lever is soft. Brakes work but soft.
Do I need a master cylinder for dual disc?

It will help.

[MauiK3]

If the master cylinder is the same and another disc is added, the lever will travel farther to move enough fluid into what becomes double the volume in the calipers. By "soft" do you mean it travels more?

[scunny]

Are you sure the brakes are bled properly ? sometimes you need to suck the air out of the caliper bleeder, a 500cc syringe does the trick .

[70CB750]

^^^ what he says.  The syringe trick is great too.

Air in brake lines is by far the most common reason for spongy brakes.  Try to tie the brake lever half way to the handle with the MC lid off - leave it overnight.  The bubbles will travel up.

Dual brakes with splitter are harder to bleed too.

[CBR954AD]

hey 70CB750,
i have bled the brakes over 10 times, using just a tube and can, then i actual brake bleeder, i also tried to hold up the MC in the air to make sure i get any bubbles out in the main line, and be the highest point of the system while bleeding. there is no air in the system it has been verified, i get solid fluid drain from both calipers.
when i have the calipers removed i squeeze the lever and the pistons are closing as they should. but its just very very soft. I forget to mention i also have Gulfer SS lines, and splitter. the MC and lever are from amazon. i paid like 30 bucks for it. should I buy a MC for dual disc setup?

[70CB750]

How did you verify it?  Can you see inside the brake hose? 

[RAFster122s]

What size piston is on the master cylinder you bought for $30?

[RAFster122s]

Gaffer SS lines?  Are there two lines from the splitter down to each caliper?

[CBR954AD]

Yes single line from MC to splitter, then 2 lines. Left and right. The MC and lever are 5/8 short lever universal I'll send pic in a min

[RAFster122s]

What is the master cylinder piston size? That will be a large determining factor in your brake lever feel. If it is too small you will have too muchlever travel whereas too lage leads to wooden feeling without much lever travel.
So, can you tell us the bore size of the MC?
David

[scottly]

Yes, the original MC will work with dual disc setup.  I used it like that on the K0 with dual disc.

Also used early K MC on 78F front brakes, works great.

I've also had good results with a stock 14mm MC, with both 38mm early calipers and late K 42.8mm calipers.

[scottly]

Yes single line from MC to splitter, then 2 lines. Left and right. The MC and lever are 5/8 short lever universal I'll send pic in a min

5/8" is 15.875mm. Kawasaki used a 5/8" MC with dual 42.8mm caliper pistons, so unless your GL calipers are even larger, then it's likely your soft lever is due to trapped air.

[CBR954AD]

sorry guys not sure what the piston size is. i can say 100% there is no air in the system, I have bled system 50 times. i have solid flow when i open the bleeder on each caliper, one at a time as procedure. but lever is very soft, no matter what i do.

[scottly]

How close to the grip does your lever come when you squeeze it hard? With a 14mm MC and dual 42.8mm calipers, my lever is about 1" from the grip. It only takes a tiny air bubble to affect the lever feel.