Topic: Dual Disk - Larger Master Cylinder - Confusion

[ev0lve]

Currently I have my bike set up with dual disks and the stock CB750k master cylinder.

I had it in my head that a stock CB750k master cylinder piston is 11mm and the caliper piston is 38mm but poking through the shop manual I find this on page 141



which indicates 14mm for the MC. Confusion then sets in as a 14mm piston "should" be sufficient for reasonable performance as this chart indicates

http://www.vintagebrake.com/mastercylinder.htm


but it's not and has a pretty long pull when fully bled.


I've read that I'm looking to pair up an MC and set of calipers that will yield a 12 - 14:1 ratio. My brain has gone soft here though and I think I only need to step the MC up to a 15mm piston to get what I want out of my brakes.

Is 12:1 not the magic ratio? Is this chart wrong? Would someone apply some brain cells here and double check that makes sense?

Thanks

Edit:

There's my damage. Now I'm even more confused...

"Single piston calipers are much happier in the 14:1 to 12:1 range."

"For 2 piston opposed calipers, I like ratios in the 27:1 range, feeling some line and caliper flex. For a firmer lever, use 23:1."

So... to get the ratio this fellow recommends I actually want a smaller piston? Am I misreading this chart and I actually want the numbers from the single caliper line? If the (2) indicates 2 pistons on one caliper then this chart is useless?

WTH? Help please 

[WHALEMAN]

I have a dual disk with a stock Master cylinder, the whole system is stainless line, and both have a speedbleeder. My lever is just as hard as most single discs with rubber lines and just a little less than perfect bleeding. Dan

[cafe750]

I'd say you've still got a bubble in the system somewhere Iggy...even with a stock master, you should get a decent feel at the lever, and be able to smoke that front wheel...

[ev0lve]

I have a dual disk with a stock Master cylinder, the whole system is stainless line, and both have a speedbleeder. My lever is just as hard as most single discs with rubber lines and just a little less than perfect bleeding. Dan

Same here  with the exception of the just as hard as most single discs. It does start to grab almost immediately but just doesn't stop the bike fast enough even at full grab which was kind of why I did the duals in the first place.

To be fair the front brake was hung before I rebuilt it so the only comparison I have is with previously owned motorcycle brakes.

I'd say you've still got a bubble in the system somewhere Iggy...even with a stock master, you should get a decent feel at the lever, and be able to smoke that front wheel...

Been wondering that. Took a lot of care and time bleeding them but I can always do it again.

Would either of you take a pic of your levers maximum travel?

Also, any thoughts on the table above as to what ratio/size would be optimum for two 38mm caliper pistons?

[754]

I found very interesting what fishead had posted on the last thread (asking about dual disc). Post #9 on thread by kre-eper.

He was talking about using bigger M/C off a GL, and then going back to the smaller one.

[ev0lve]

I found very interesting what fishead had posted on the last thread (asking about dual disc). Post #9 on thread by kre-eper.

He was talking about using bigger M/C off a GL, and then going back to the smaller one.

Saw that (trying to answer that guys question is what spurred this one).

http://forums.sohc4.net/index.php?topic=50341.msg533227#msg533227

And the Randakk's article it references
http://www.randakks.com/Brake%20Parts.htm

No wonder GL1000 brakes feel "wooden!" After some experimentation, I've found a killer improvement for the GL1000. A modern Nissin 5/8" master cylinder absolutely transforms GL1000 braking to near-crotch rocket levels. This unit dramatically reduces lever effort and provides significantly improved feel. The lever is still quite firm, but not "wooden." Max braking near the threshold of lockup is much more predictable and easier to modulate. Best of all, I can now use 2 finger braking on a GL1000!

(I also tried a 14mm unit, but it was a bit over-leveraged. It transmitted a bit more line and caliper flex and felt too "squishy" for my taste.)



So a 5/8ths? What am I not understanding about the ratios in that chart?

Also, what's the piston size on a GL1000 MC just for reference?

Edit: I love the internets  MC off a Goldwing 75 - 78 is 17mm and 16mm-ish for the 79 and it has 38mm caliper pistons as well.

[lordmoonpie]

I can't recommend classic AP master cylinders enough - superb feel and power. If only I could bleed the flaming stock honda calipers I could tell you how they work on a 750! On a 500cc Black Bomber I can lift the back wheel easy with on efinger and that's also a single disc!

http://www.apracing.com/info/products.asp?product=Classic+Front+Master+Cylinder%2DCP2215%2D90_2557_1875

[SOHC4 Cafe Racer Fan]

Simon,

Are you using the AP master cylinder on your CR750?  Is that with the AP Racing calipers? 

I wonder if you'd get even better braking with the EBC Rotors?

[ev0lve]

I can't recommend classic AP master cylinders enough - superb feel and power. If only I could bleed the flaming stock honda calipers I could tell you how they work on a 750! On a 500cc Black Bomber I can lift the back wheel easy with on efinger and that's also a single disc!

http://www.apracing.com/info/products.asp?product=Classic+Front+Master+Cylinder%2DCP2215%2D90_2557_1875

Nice (and expensive  ) Thanks for that link.

So you'd go 16mm?

I dig the adjustable 12 - 19mm MC. Just dial it in!
http://www.apracing.com/info/products.asp?product=Original+Adjustable+Ratio+Master+Cylinder%2DCP3125+Family_2551_1095

[fishhead]

And the Randakk's article it references
http://www.randakks.com/Brake%20Parts.htm

No wonder GL1000 brakes feel "wooden!" After some experimentation, I've found a killer improvement for the GL1000. A modern Nissin 5/8" master cylinder absolutely transforms GL1000 braking to near-crotch rocket levels. This unit dramatically reduces lever effort and provides significantly improved feel. The lever is still quite firm, but not "wooden." Max braking near the threshold of lockup is much more predictable and easier to modulate. Best of all, I can now use 2 finger braking on a GL1000!

(I also tried a 14mm unit, but it was a bit over-leveraged. It transmitted a bit more line and caliper flex and felt too "squishy" for my taste.)

Randakk is also using the same 5/8 master cylinder on his new brake set up with dual twin piston calipers, and it's the same master cylinder that vastly improved the stock GL 1000 braking system. He went from 2 large pistons to 4 smaller pistons and is using the same master cylinder for both.
 About 3/4 down the page
http://www.randakks.com/Randakk's%20Endurance%20Special.htm

Here's his review on them

http://www.nakedgoldwings.com/forum/viewtopic.php?t=9496&highlight=

 Getting a larger GL 1000 master cylinder willl just give your CB dual disc the "wooden feel" that the GL 1000's have (you will pretty much have a dual disc system that is basically the same as the stock GL 1000).
 Using a stock CB master cylinder on your dual discs will give you slightly more travel in the lever but I find the extra travel when the brake are being applied help in making the brake easier to modulate. I've never had a dual disc CB with the stock CB master cylinder bottom out on the grip.
  If you are doing a brake up grade like the dual disc, it is wise to also upgrade the brake lines to braided stainless steel. Getting rid of the flex in the stock rubber hoses is a large improvement on its own, even with a single disc.
 Newer master cylinders like Randakk's usually have 4 or 5 position lever adjusters on them. The adjustments on the levers allow you to place the active braking point to a comfortable position on your fingers. Having the active braking point where your fingers are uncomfortable and too far from the grip will give you the "wooden feel". Being able to adjust the lever to be at a comfortable  "squeeze" for your hand will improve your braking action and your braking confidence.
 I'm using a 5/8 master cylinder identical to Randakk's on my dual disc 6 piston set up and it works fantastic.

[lordmoonpie]

Simon,

Are you using the AP master cylinder on your CR750?  Is that with the AP Racing calipers? 

I wonder if you'd get even better braking with the EBC Rotors?

I've got the AP master cylinder but stock calipers for now as I want the authentic CR look for at leats the first season. Plan to switch to AP calipers afterwards so I can stop properly. No EBC rotors for me though - custom made cast iron 290mm rotors made by CAT Engineering. They take a stock CB750 rotor, machine off the centres and add their own rotor material. Beautiful work and judging by the Bomber, they'll grip like sh*t to a blanket 

[736cc]

 Simply swap a master from a bike w/ dual discs would seem an ez fix. I'm running clubmans which has a drop angle so a stock master cap would never be parrellel to ground (try filling and bleeding in that situation). A 1982 -86 V45 Magna master has the perfect angle for clubmans and I'm very satisfied w/ braking feel. SS brake lines are a must. Polishing caliper pistons and super-clean caliper bores should be on your list as well. Sanding discs and pucks THOROUGHLY  w/ 150 grit sandpaper a racer trick. (Yes, believe-it-or-not, sandpaper works great. Shops also use extremely coarse scotchbrite on an air tool) Bleeding duals is tricky, furthest caliper 1st, tap the lines, tap the master, tap the splitter, to get bubbles moving out. After getting a decent firmness, pull the lever in TIGHT and zip-tie it overnight. Then bleed again for ieven more mproved results.
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[Industrial Cafe]

the late 70's Kawasaki KZ650 looks like our master cylinder, and is designed for dual discs.


this auction is over but it could help.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=140310638267

[mystic_1]

Thats good info, IC.  Looks fairly identical to a stock 750 master.  Wonder if they were originally made by the same supplier.

mystic_1

[SOHC4 Cafe Racer Fan]

Simon,

Those rotors are serious!  Very cool.
On my next project, I'm going with dual AP calipers.  I'm not sure on the MC, and I'm wondering about the EBC discs.

[754]

Looks like a Honda MC with a different splitter..
prety sure kawi was a bit diff, cap  was for sure..

[Industrial Cafe]

i've been tricked

[ev0lve]

Alright. You all have convinced me that the stock MC should be better than it is. So while I have to wait for the 5/8th I'll bleed the brakes again - this time with the pressure switch off so I can get as straight a shot from the calipers to the MC as possible I think.

Wish I'd known a little more going in. I think I would have run individual lines from the MC to the calipers in retrospect.

And really? Overnight?

Thanks all!

[mystic_1]

Do you know this one?

With the master cylinder cap off, S L O W L Y squeeze the lever all the way down (so as to avoid jetting brake fluid all over the garage) and hold it for a second, then... FLICK let it go suddenly so that it springs back and snaps open.  You'll probably be able to observer a small stream of very fine bubbles rising out of the bleed holes inside the reservoir.  Repeat this several times.

Also, flick, twist, bend, tap, vibrate the lines to help work bubbles up to the top.

mystic_1

[ev0lve]

Thought I did everything I could (flicking, tapping, rubber banding the lever, swapping the bars back right and left etc  ) EXCEPT letting the pressure switch loose off the bike so as to be able to straight line (sort of) the hoses and letting it sit over night. I only spent a couple 3 hours on it  There is a loop coming out of the switch to the MC and that's about the only place I can imagine it being unless it's in the high side of one of the calipers (bleed screw on the top side).



Just have to give it a try!

[mystic_1]

For sure loosen the T fitting at the brake switch, air just LOVES to get caught in there.  Twist it about and try to turn it side to side and upside down even if possible.  Teach it who's boss!   

Good luck!

mystic_1