Topic: replacement of master cylinder - what alternatives can i use

[superchode]

i tried answering this one for myself by searching the archives - but it seems you clever lads have used master cylinders from all kinds of different bikes on your CBs, and i'm not sure what will work best for me.

i've got a '78 CB750F on which i rebuilt the master cylinder using a rebuild kit. i was trying to get my brakes online last night and while bleeding the fronts i noticed that a small amount of brake fluid was making its way past the wrong side of the master cylinder and to the brake lever.

the master cylinder is ratty. i'd love to replace it and i really don't want to spend any more time and/or money to bring it back from the dead (especially after failing the first time).  what can i bolt on to replace it.  i'd like it to look nice (i think the stock MC is a bit ugly) - but mostly i want it to work properly without modification, if possible. i'd also like a fairly easy time of hooking the sensor switch up to the tail light circuit.

any suggestions? i'd like to start scouring ebay soon so i can get the process rolling. i'm only a couple weeks away from getting this beast back on the road otherwise (just doing some bodywork).

[kaceyf2]

Hya there bro, I have a 1978 F2, and this had ( as stock) one of those "square" master cylinders up front, which as well as darn ugly, also have a habit of leaking due to bad deseign. The square top is held down in the corners by a bolt in each corner, and with time and ageing they tend to Bow in the middle, and leak. So, i got one from a dohc cb750/900 , they are capable of operating the two calipers up front, and have a round top thats much better deseigned.  I havent tried for myself, but I believe that a round master cylinder for a single disc cb750 sohc will NOT operate Two discs,like I say, I have not TRIED to verify this myself, just heard off others and not wanted to purchase one that was wrong.
hope this helps.

[superchode]

yeah, that helps.

ideally, i'd like to replace it with a M/C from a newer bike so that i can find one salvage or otherwise that's in pristine shape and it's just a matter of bolt-on and ride.

that said, i suppose i could look around for a DOHC replacement that's completely functional.

in hindsight i shouldn't have tried to rebuild this thing... it's obviously seen better days. i figured new internals would solve it, though.

[hevykevy420]

I used a m/c from an '02 Honda RC51 'cause i had an extra laying around from when i crashed by RC.  just bolted right on.

I must say, it kicks @r$e on the CB, especially combined with stainless steel brake lines, but I had to lose the right side mirror as it screwed into the old m/c.  (i'm always in the slow lane anyway here in Hell, TX USA.)  Much better caliper "feel" and more confident riding on the interstate with all the monster trucks out there...

Raised fluid container looks a bit funny but oh well...not too bad...more about function to me anyway...

[puppytrax]

...So, i got one from a dohc cb750/900 , they are capable of operating the two calipers up front, and have a round top...

...??...What year DOHC CB750/900??...All the ones I've looked at had the rectangular top...

[eurban]

I don't think that Kacey is correct about his 78F  master cylinder being stock unless perhaps it is non US model.  Both the 77 and 78 Fs (dual discs) used a MC with a round aluminum resevoir.  This looks just like the standard  SOHC resevior except perhaps having a bit larger resevoir and having a larger bore for the dual discs.  Get a MC sized for a dual disc appilcation.

[Gabus]

I too have a F2 but my M/C is fine..so far. I did recently add SS brake lines it did a lot to improve brake feel. I asked my local Honda dealer about handle bar diameter on Goldwings. He said though 1500..maybe 1800 they used 7/8" bars. Doesn't look too bad IMO runs 2 calipers, readily available on ebay.

[Honda CBer]

the 79-83 DOHC bikes have the square ones, as does my '79 CB650... there were different ones, though... for example, 750C had only one disc up front, and had the brake line exiting the m/c from the left side, much as a cable would... 750F had two rotors, and the hose exited the m/c towards the front of the bike... later models had dual-piston calipers...

[Noel]

I'm using one from a late model VFR. I think as long as you find one that is set up for double disks you'll not go far wrong.

[superchode]

k, found one from a 5th gen. VFR.  will i need to get a new line made up to run from the M/C to the splitter inbetween the forks - or should i be able to get by with the old line?

[Noel]

IMO, braided lines are one of -- if not the -- biggest improvement you can make to the brakes.

As I see it, your options are

A) Reuse the existing lines, which will probably result in "better" not  "best" performance.

B) Go to the trouble and expense of having a set of custom line made up, which should result in great feel but may or not be terribly affordable or convenient

C) Bypass the splitter entirely  and use the existing brake light switch in the VFR master, along with a set of stock braided lines from Ebay, which should result in the best possible performance but may give you some headache finding -- and possibly returning -- stock brake lines. You'll also need to buy some wire and crimp connectors to hook up the switch.

I went with option "C" myself and it worked out fine. Stock Galfer lines for a late model CBR1000RR, 25" long, fitted right up with included banjo bolts and washers. But my bike's got clip-ons, and those lines might not be long enough if you're running the stock handlebars.

Having said that, I see no reason why solution "A" isn't perfectly workable, assuming your existing line is in satisfactory condition. You *may* have to find a banjo bolt for the VFR cylinder: the threads on your SOHC banjo bolt will fit but the size of the holes and overall lengthmay not be ideal. I don't know for sure.

Hope that helps.

[nickjtc]

You sometimes see generic, but brand new, brake assemblies on eBay. I'm not sure whether there 'one size fits all' systems will provide the necessary hydraulic force for all applications, but it is worth a look.

[Bodain]

I rebuilt 3 70's Honda's. In all cases I rebuilt existing old master cylinders. Now that I'm working the Yamaha RD, I've learned a valuable money saving lesson.  It cost $41.00 for the front RD master cylinder rebuild kit. I said enough. Started browsing Ebay.Almost any front master cyclinder for a 7/8 in. bar will work. Total spent for a fully functional 1995 Kawasaki master cyclinder = $13.00
So unless you really must maintain the original for whatever reason. You can save mucho bucks.

The RD currently has GSXR Clutch Perch and lever, left handlebar electrical controls, rear master cylinder. You can just get these newer items in almost perfect shape for a fraction of old original beat up parts.

Now that I'm tuned in. There are a few parts on the CB 750 that will be replaced with non Honda parts.

[nickjtc]

Useful to know..

[superchode]

i definately wish i hadn't bothered trying to tune up the original m/c... especially considering that it just looks awful.

hopefully i'm on the right path now - no ss lines just yet... i can always get to that over the winter. just trying to get a couple months on the road out of this bike before the snow flies.

[kaceyf2]

Yep. should have said my cb750 1978 f2 sohc master cylinder was indeed square as stock, but it is a Uk one, i did not realise the usa ones are different, However, many people changed the square ones over the years to round ones, for a variety of reasons, If you bought your bike in the Uk secondhand, and it had a round one on it, it would be an easy mistake to think it came as "stock"
Hope you have got it all sorted anyways.