What alternate brake master cylinder will work?

1985 GPZ750. The stock front brake lever and master cylinder are trashed. Im going to start over with an aftermarket one or another from a diff bike. Anyone know what other ones will work?

Or...what the piston and size of the oem one is?

single disc set up uses a 14mm or 9/16, dual discs use 16mm or 5/8s
Pete

If it's dual disc, it's a 5/8" or 16mm.

Any 5/8" bore bar mount (7/8" bars) will work

Thanks.
What do more modern dual disc brakes on USD forks use? Let's say I go to a GSXR USD front end. Do they still use the 5/8"?

Or

What is the effect of using a 14 or 13 mm bore master cylinder with the stock GPZ brake system?

taylee wrote: What is the effect of using a 14 or 13 mm bore master cylinder with the stock GPZ brake system?

Touchy brakes that require a long pull on the lever. Two calipers require twice the amount of brake fluid movement compared to a single caliper. So the piston in a small bore must move further than one in a large bore to push the brake pads the same distance. Ed

Pretty good explanation. So, I'm thinking going to a 14mm from a 16mm wouldn't be too bad. Meaning, the lever likely wouldn't go all the way to the bar.

Any idea what the modern dual disc bikes use?

Tokiko 6 pots are pretty common.....or slightly upper market Nissin components.......of course Brembo would be really cool to have or for the ultimate....ISR (Swedish stuff - read - very expensive). I picked up a 97 Yamaha racing 750 front end that has Sumitomo calipers on it, I had never heard of them but they seem to be well regarded.
Hth

taylee wrote: Pretty good explanation. So, I'm thinking going to a 14mm from a 16mm wouldn't be too bad. Meaning, the lever likely wouldn't go all the way to the bar.

Any idea what the modern dual disc bikes use?

I wouldn't bet on it. Dual disks use a 5/8" bore MC. 5/8" = about 16mm. Single disk brakes use 14mm. I don't understand why anyone would want to use the wrong size MC when trying to upgrade their brakes. It appears that you didn't get the answer you wanted, so you chose to ignore it.

The different bore sizes in the brake systems used by Kawasaki and other bike manufacturers were engineered for performance - not looks or cost savings, so there really was a reason for the different bore sizes. Assuming you are upgrading your brakes to a dual disk setup it makes sense to me to employ the same master cylinder size specified by the engineers.

Here's a cut & paste from another site you may find interesting. This KZ650 owner tried a 14mm master cylinder with dual disks and came up with what I consider to be a rather conclusive test.

"I thought I was going crazy not being able to get a decent lever feel no matter how I bled the brakes. Done the reverse bleed and absolutely no air bubbles coming up to the master. I then started to wonder if it really could be the dreaded 14mm vs 5/8" issue so started clamping down hoses to see if reducing the volume allows the master to operate at its sweet spot. Before starting the lever could easily pull all the way to the bars. Clamped the single line from master to splitter and the lever was rock solid. Moved only about 15 mm from the resting point. Released it and back to the full travel with mushy feel. Then clamped the left caliper hose and this time the lever comes in about 25 mm and is pretty firm. At that point I thought I may have air in that caliper but swapping the clamp to the right side hose has the same effect, a near perfect feel with 25mm of travel before brake is fully on.

This leads me to believe that the m/c simply cannot move enough fluid to operate the brakes through these hoses, splitter and dual calipers."

If you really want to wreck your GPZ, using the wrong size is a great idea. Modern duel disc bikes use 5/8 in. front mastercylinders.

taylee wrote: Pretty good explanation. So, I'm thinking going to a 14mm from a 16mm wouldn't be too bad. Meaning, the lever likely wouldn't go all the way to the bar.

Any idea what the modern dual disc bikes use?

Here's another approach - turbo charge the bike and remove the front brake entirely. :woohoo:

Just in case any questions remain...

Using a 14mm (or 1/2") master on a dual disc KZ will result in a spongy feel at the lever. For any given level of hand pressure though there will be more braking force. That's the tradeoff.

Some modern bikes with dual disc setups have masters that are smaller than the 5/8" that came on our bikes, but those bikes have ultra rigid brake systems so the lever feel is firm despite the small piston size.

On my KZ750E I used an EX650 master (5/8" bore). You can find these masters dirt cheap on ebay, and since they are late model, you don't have to worry about internal corrosion. Only downside is the reservoir angles down a little more than ideal (in my opinion) for use with the common superbike end handlebars.

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