Topic: MC rebuild or modern replacement

[stevenmgrr]

My master cylinder on my '71 CB500 K0 is now leaking.  I am getting fluid escaping from the piston area under the lever.  After reading several post after doing a search, I am wondering if it would be better to replace it with a 'modern' unit and avoid the hassle of fixing.  It appears that I can purchase one off a modern bike for about the same price as a rebuild kit.

What modern units will work?

Thanks

[hopterfixer]

I just replaced my factory '74 750 MC with one from a 2001 Kawasaki EX 500.  It has an adjustable lever and the pull is not at hard as the original.  I paid $25 on Ebay and am very pleased with my purchase.

[stevenmgrr]

Can I use a '75 Goldwing master cylinder?

[Shenanigans]

It would work fine if not better.

[333]

If you decide to go with something else other than stock, do the next owner (and maybe even yourself) a favor and document what ever you use.  All parts go bad sometime, and rebuilding it will be much easier when you know what it came off of.

[stevenmgrr]

I have read several posts with guys using all different kinds of master cylinders from various size bikes and even different manufacturers.  Is there any criteria that you use to determine what will work or can you just pick up anything that is a good deal?  Do I have to change out the rest of my braking system, such as the caliper?  I notice that some of the newer units have the brake light switch in the master cylinder unit itself.  Mine has the switch below, on the tree.  Will I have to do any special modifications?

From what I have read, rebuilding mine can be a real PITA.  I don't what to have a new project replacing my entire braking system though.

Thanks

[MCRider]

I have read several posts with guys using all different kinds of master cylinders from various size bikes and even different manufacturers.  Is there any criteria that you use to determine what will work or can you just pick up anything that is a good deal?  Do I have to change out the rest of my braking system, such as the caliper?  I notice that some of the newer units have the brake light switch in the master cylinder unit itself.  Mine has the switch below, on the tree.  Will I have to do any special modifications?

From what I have read, rebuilding mine can be a real PITA.  I don't what to have a new project replacing my entire braking system though.

Thanks

My experience has been its hard to go wrong as long as the replacement is not in need of rebuildiing and the other obvious things like switch clearance issues etc.. I've just grabbed one out of a box at Cycle ReCycle Part II a couple of times, a Kawasaki of unknown ID, and one off of a VFR. Both worked fine.

[Soos]

If you decide to go with something else other than stock, do the next owner (and maybe even yourself) a favor and document what ever you use.  All parts go bad sometime, and rebuilding it will be much easier when you know what it came off of.

+1!!


I have a "little black book" that I keep notes on all mods, when I made that mod, and what bike it came from.
Without it, in the future, i'm sure I would get lost trying to get repair kits.
Besides that I tag and keep the origional parts just in case i do ever sell the bike the new owner will have the option of going back as close to stock as I received the bike in.

l8r

[ieism]

The brakeswitch is not an issue, you can still use the old one. Or if you prefer you can rewire to use the one on a new MC.
The only thing to consider is the MC bore size. so don't get a unit from a new sportsbike with dual disks and six pot calipers and expect it to work.
18 to 19 mm works on my 550 with dual disks. The lever feels different, you have more feel in how hard you are braking.

[alltherightpills]

The only thing to consider is the MC bore size. so don't get a unit from a new sportsbike with dual disks and six pot calipers and expect it to work.
18 to 19 mm works on my 550 with dual disks. The lever feels different, you have more feel in how hard you are braking.

Here is a chart with the ratio's between mc bore and caliper piston size:

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

I believe our pistons are 38mm

[Steve F]

If you decide to go with something else other than stock, do the next owner (and maybe even yourself) a favor and document what ever you use.  All parts go bad sometime, and rebuilding it will be much easier when you know what it came off of.

I used a front master cyl from a '77 CB750F, and I'm having a tough time trying to locate a rebuild kit for it.  Any ideas or suggestions?

[stevenmgrr]

The only thing to consider is the MC bore size. so don't get a unit from a new sportsbike with dual disks and six pot calipers and expect it to work.
18 to 19 mm works on my 550 with dual disks. The lever feels different, you have more feel in how hard you are braking.

Here is a chart with the ratio's between mc bore and caliper piston size:

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

I believe our pistons are 38mm

I am going to run just the single stock caliper.  Can I use a 14mm or a 12.7mm master cylinder?  Which one will give me the same or better feel?  According to the chart mentioned above, going to a 12.7mm master cylinder bore will increase the ratio to 10.03 over the 14mm's 7.37 ratio.  Can someone describe what this translates to in feel and braking power?  I would assume that a slightly higher number would be better?

[ieism]

That chart suggests an ideal ratio would be 14:1 to 12:1. It also suggests our bikes have the wrong stock size, and will get worse with a bigger MC bore.
I'm not sure I agree with that.

Forget the chart. I've seen tons of posts here saying that a 14mm worked great on their 550's with a single disk. Never have I seen a post where a newer 14mm caliper didn't work.

[eurban]

I would tend to disagree.  Do not use a master cylinder sized for a dual caliper bike like the GL1000 on a single caliper setup.  My experience with the single caliper setups is that they require a very good amount of squeeze in order to get any significant amount of braking force.  A larger bore master cylinder will mean that you will have to squeeze even harder.  . . .If you are going non stock, get an MC sized for your application!

[ieism]

Then you go find me a post where a 14 mm didn't work on a 550. In fact, I've seen posts where a gl1000 MC worked fine with a single disk.

I trust people on this forum more than some chart somebody made. Not all motorcycles are alike.

[fishhead]

  Heres a link from the goldwing guru (www.randakk.com) who puts 5/8 master cylinders where a stock 11/16 master cylinder was (dual disc). The GL 1000 also uses 2 X 38mm pistons in the dual disc calipers (1 piston in each caliper). The stock 11/16 master works "fine" but the smaller 5/8 master cylinder gives a better braking feel and, in turn, works better.
 A larger master cylinder will work "fine" in most applications, but a correctly sized master cylinder allways works better.

http://www.randakks.com/Brake%20Parts.htm

[eurban]

Then you go find me a post where a 14 mm didn't work on a 550. In fact, I've seen posts where a gl1000 MC worked fine with a single disk.

I trust people on this forum more than some chart somebody made. Not all motorcycles are alike.

I am not basing my suggestion on "some chart."  I have F'd around with a good number of master cylinder and brake combos on a number of bikes.  It really is a matter of physics as a larger diameter bore will give the lever less mechanical advantage making the squeeze more difficult. Hydraulic principles don't care what brand of bike you are using.  This is why such a chart actually is useful.  As I also mentioned, the front brake on your typical SOHC requires a good amount of lever effort to get a "decent" amount of braking.  "Improving" your front brake by installing a MC with less mechanical advantage than stock is a bad idea. . . .I am sure that a properly functioning MC from a GL1000 will work better on a single disc bike than a stock one in need of a rebuild. . . . Sorry, but I trust my own actual experiences (particularly when they are backed up by science) more than reading about what others are satisfied with.

[stevenmgrr]

Update,

I replaced my old master cylinder with a generic modern unit off ebay.  What a big difference!  Definitely and improvement on both braking power and feel.  I am also planning on upgrading my brake lines to braided lines. 

[lordmoonpie]

Just replaced mine after messing about with the rebuild kit on the original one. After a week of PITA fiddling it's back together and still rubbish. Now I've got a classic AP lockheed master cylinder so it doesn't look modern and fits with the style of the bike and guess what! I still can't bleed the flaming things even though the calipers have been rebuilt with new pistons and seals!!

Bleedin' brakes....I hate 'em! 

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

[lordmoonpie]

Yes it's real nice stuff the AP kit and I use it on all the race bikes. The Two VTRs are modern fully adjustable stuff but the Bomber has the same master as I'm trying to bleed out now.

Since you mentioned it I'm strting to recall that I couldn't do the Bomber either until I took the brake calipers off and bolted them to the bench and bolted the master cylinder to a handlebar in a vice below. Problem on the 750 with dual discs is it's a nightmare to get the calipers off - 40 minute job just to remove them. Got to have a go though, it's better wasting 40 minutes than not being able to stop 

Thanks for the reminder Fishhead, I'll work on it!!

[MoTo-BunnY]

I have used a 14mm master cylinder that works for either single or dual disks, for over 2 years and many miles - works great.

(the below is a copy/paste from old posts I have made a few times now - since this keeps coming up)

http://forums.sohc4.net/index.php?topic=28649.msg294679#msg294679 (see this thread)

I replaced my old stock master cylinder with one of these eBay units - just like the one starting this thread. It was one of the single best improvements I have done yet - BIG improvement in braking.








If you want to upgrade your master cylinder to one that will support one OR two discs (and REALLY improve your braking), you can get this style off of feeBay:






This is apparently a very close copy (and a very good one, at that) of the Nissin master cylinder? (it could even be made by them but there isn't any manufacturer markings anywhere on either of the ones I bought) This company sells them and I have bought and installed two of them on two different bikes ('73 CB500 & '79 CB750K) with fantastic results: The style I used were 14mm piston - works great for our Hondas - the stock brake hose and mirror even fits the threads - and the brake switch hooks right up if you extend the wires and change the wiring connectors)

usa-motorcycles-inc

However, they seem to be running fewer of these at auction than they used to and are jacking up the price. (I bought both of mine at $39.95 each - now the same style is $49.95+)

You don't have to be particular with what brands they post in the title as long as its the 14mm piston model and it looks like this picture. .  .they are all the same. I have noticed bidding wars over $80.00 on one that says "VTX" or Shadow or something while the exact same part in a different auction goes for beginning bid ($49.99) . . . . lol.

[mrbreeze]

I just added a Yami 650 master from Mike's XS.Works great.I also used the left & right hand controls. Looks good...works good....case closed.

[Triffecpa]

not trying to get too deeply involved in the replacement size discussion, but if your MC piston is too large, the brake lever will have very little travel and it will be hard to pull.  It makes it harder to modulate the brakes.  If your piston is too small, then the lever will be easy to pull, but will likely pull back too close to the handlebar.  Either way, there is a fixed amount of fluid that needs to move to move your given brake caliper piston.  You can use a large piston and not move it very far, or use a smaller piston and move it further (ie more travel at the lever).

Tracy

[bert96]

Can I use a '75 Goldwing master cylinder?

That's what i have and i replaced my old rubber brake hose with stainless steel brake line(black) and vtr 1000 rotors,a little bit of old and a little bit of new

[Joksa]

Well stock master cylinder size is 14mm and the cb750k4 caliper piston diameter is 38mm (I just went and measured my old one just for you)

The hydraulic ratio being the important thing so

All Stock
38^2/14^2 = 7.37

Stock master cylinder and dual front calipers.
(2*38^2)/14^2 = 14.73

16mm master cylinder and dual calipers
(2*38^2)/16^2 = 11.28

From the chart at http://www.vintagebrake.com/mastercylinder.htm it says that single piston calipers like a ratio of 14:1 to 12:1. The stock ratio is well below this meaning that you have to pull the lever harder but not as far to get the same braking force. A larger ratio means that you have more lever travel and less effort to brake. You probably want something from 14mm to 16mm. Personal preference maybe. Pads and lines make a difference too.

I suspect one of the biggest reasons people notice braking improvement when going to dual disks is the master cylinder ratio changes.

These guys have all the sizes of Brembo master cylinders you could need plus they also sell things like aluminum banjo bolts and custom braided lines at exorbitant prices. The regular master cylinders seem to be priced pretty reasonably though. It's where I bought my stuff.
http://www.yoyodyneti.com/Category.aspx?CategoryID=3006

*Disclaimer: I ignored Hondaman's advice recently when I purchased an 11mm master cylinder for my single disk setup giving me a ratio of about 12:1. I haven't really ridden it yet so hopefully it doesn't kill me. I haven't actually done anything involving dual front calipers.