master cylinder swap

hello can anyone tell me if i can use a front master cylinder from a 76 900 with single disc brakes on a bike that has dual front disks

rlcarter wrote: hello can anyone tell me if i can use a front master cylinder from a 76 900 with single disc brakes on a bike that has dual front disks

May result in the brake being overly sensitive
This is why Kawasaki increased the single disc 14mm m/c to a 5/8" m/c on dual disc models.

For the single disc Z1 with stock 14mm m/c, Kawasaki offered an optional dual disc kit from the factory. In addition to its other components, the kit included a larger 5/8" master cylinder.

Good Fortune!

Patton wrote:

rlcarter wrote: hello can anyone tell me if i can use a front master cylinder from a 76 900 with single disc brakes on a bike that has dual front disks

May result in the brake being overly sensitive
This is why Kawasaki increased the single disc 14mm m/c to a 3/8" m/c on dual disc models.

For the single disc Z1 with stock 14mm m/c, Kawasaki offered an optional dual disc kit from the factory. In addition to its other components, the kit included a larger 3/8" master cylinder.

Good Fortune!

Hi Patton - the other way around - the master cylinder doesn't move enough fluid, so there's a risk the lever can come all the way back to the bars.

Jeff, was 3/8" a typo? Did you mean 5/8" for a dual disc front M/C?

BohicaBob wrote: Jeff, was 3/8" a typo? Did you mean 5/8" for a dual disc front M/C?

Correct. Dual disk brakes use 5/8" master cylinder. Ed

The typo was mine , which has since been corrected.

The question posed is "can anyone tell me if i can use a front master cylinder from a 76 900 with single disc brakes on a bike that has dual front disks?"


www.kzrider.com/forum/5-chassis/476106-brake-upgrade#476106

531blackbanshee wrote: laymans terms.
too small a master cylinder may not apply enough force to stop you.
too big may apply too much force and cause you to lock up the wheel....

steell wrote: ...
You do realize that you have that backwards?
Smaller master cylinder = more force
Larger master cylinder = less force
At the caliper.
5/8" = ` 15.5mm
. . .
14mm would make your brakes touchy, as in one finger lock up. 15mm would be much better....

steell wrote: ... It helps when you've spent a few years working on different hydraulic systems,

To explain for anyone else reading.

I'm going to make some generalizations to avoid boring people with the math.

I'll assume you have a KZ with a single 12" brake rotor, a 50mm caliper single piston, and a 1/2" master cylinder bore. You squeeze the brake handle, you push the 1/2" piston 1/2", and you generate 500 lbs of stopping force. Now you bolt on a second 12" rotor and a second 50mm single piston caliper, but keep the same 1/2" master cylinder. Now when you squeeze the brake lever the piston has to travel twice as far (because it's feeding two calipers), and you're applying 500 lbs of stopping force to each disk, for a total of 1000 lbs of stopping force, so the front wheel locks up.

(Warning, slight exaggeration ahead)

Since locking the front wheel is not an ideal outcome, you decide to reduce the stopping power a bit, and you want to reduce the lever travel to what it originally was, so you install a larger master cylinder. So you find a 5/8" bore master cylinder, and install it. Now because of the larger diameter (and larger area of the piston) a 1/2" movement of the piston moves almost twice as much fluid, so the lever only moves half as far. And because you have increased the area of the piston, the 30 lbs of lever force that formerly was exerted on 1 sq. in. of piston is now spread over two sq. in. of piston, so where you did have 30 psi going to each caliper you now have 15 psi.
30 psi on 1 sq in vs 30 psi spread over two sq in (15 lbs per sq in). move more fluid and lose power, move less fluid and gain power. Look at a hydraulic jack, little tiny pump and big cylinder lets you lift tons with one hand, slowly.

Earthmoving equipment uses variable displacement hydraulic pumps, and often have switches to go from large displacement, high flow, high speed, low power, to small displacement, low flow, low speed, much power.

Good Fortune!

books.google.com/books?id=hRmdC6hZ2q0C&p...r%20cylinder&f=false

See Chapter 7 at page 63 -- Hydraulic Brake Systems




With proper bleeding, the front brake lever should firmly stop when squeezed before contacting the handlebar grip, even when using a 14mm m/c with dual discs.

But imho the slight increase in the lever's range of motion when applying the front brake isn't the main concern (the main concern being creation of an over-sensitive front brake when using a 14mm m/c with dual discs.)

Some folks have run 14mm m/c with dual front discs, and so far with no problems. Good for them, and may their good fortune continue.

But others using the smaller bore master cylinder with dual discs have reported that line pressure is excessive, and readily causes front wheel lock up in an emergency stop, with loss of control and resultant bike tip-over. In other words, the smaller bore master cylinder results in overly sensitive brake lever application with dual front discs.

While it might be erroneously assumed that lesser squeeze needed on the brake lever would be more desirable, it's important to remember that a harder squeeze should not tend to prematurely lock the front wheel (an undesirable "toggle-switch" brake).

Conversely, for the same reasons, using the larger bore master cylinder with a single front disc would require a harder squeeze to produce equivalent braking (the dreaded "wooden feeling" lever).

In any event, please be mindful of the above considerations, and carefully test the operational effect of any front brake system change before the next emergency stop situation arises.

And yes, maybe I'm all wet 'cause it sometimes gets to raining pretty hard up here in the peanut gallery. :laugh:

Good Fortune!

Thanks Patton, I really didn't want to type all that again :lol:


OEMs don't always get it right either, the mid eighties GM F bodies (Firebird/Camaro) have horrible brakes, while the Honda VTR1000 has a 14mm master cylinder with killer Nissan calipers, and a lot of owners swap them out.

Check out the VTR M/C with the F4I calipers, one finger lockup :lol:

I recently went from a single disc setup to a dual disc setup and at first did not change the M/C as it had not arrived yet. After hours of bleeding etc the lever would come all the way to the bar (although the brakes did work). After changing out to the dual disc M/C, whammo we haz mucho brakes and I can't pull it to the bar any more

I bought a master from eBay with a 14mm bore that was supposed to fit my dual disc setup. I bled it multiple times and couldn't get a solid pull. Ended up having to eliminate one disc to get brakes till I could rebuild my original.