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1980 CB750c brake upgrade part 2


In part 1 of the CB750c brake upgrade post, I discussed my brake caliper upgrade from a single piston caliper to a dual piston caliper from a 1985 VF500.

My next brake upgrade is the master cylinder. During my brake research, I calculated that my brake ratio was still less than perfect even with the caliper upgrade (I went from 9.43:1 to 10.45:1 from just the caliper change).

While I still haven't taken the upgraded bike out for a test ride, I decided I would upgrade the master cylinder to improve the lever feel and brake modulation. I was determined to have a nice progressive brake lever without the harshness reported by the cx500 blogger who upgraded to a dual 30mm piston caliper from the cb900. (his resulting brake ratio of 9.18:1 was much worse than mine at 12.7:1 after the MC upgrade)

I searched a few days for replacement master cylinders on the Internet but they are not easy to find because people don't usually go around replacing them the way they replace their exhaust systems. The few master cylinders I did find, either didn't have a place for the mirror and brake switch or were so expensive that I couldn't afford to consider them. There's a decent market for used master cylinders but you would need to know what size pistons came with which bike in order to make an informed decision about what to upgrade to. Most of the used master cylinders on eBay were old and dirty and had twisted brake levers.

I got lucky when I discovered generic master cylinders on ebay from seller "USA-Motorcycles-inc". The intended application is for "YAMAHA YZF600 YZF750R" and the price was very reasonable. The piston is 12.7mm or 1/2" and he seems to sell only 4 or 5 unique master cylinders for different applications. I'm not sure how long he'll continue selling these nice pieces or if I can get replacement levers but I'll take this for now. The angle of the reservoir and the length of the brake lever are very close to the original. The one piece unit is much nicer than the old plastic reservoir with leaky o-ring.

Comments

  1. UnknownDecember 15, 2009 at 4:27 PM

    I concur with all of your research/comments. I own a CB900f which originally had single piston dual calipers. The feel was extremely harsh and braking unimpressive. Thinking about the relationship between master cylinder piston and caliper pistons I figured I Could dramatically improve efficiency by simply bolting on another pair of single piston calipers underneath the originals,BUT still using the original master cylinder. It meant making up a pair of caliper brackets and extending the hoses, braided of course. Well the results were OUTSTANDING. The feel is soft but as you approach the handle bar you get the feeling that you could bend the forks under you. the braking is about as good as my sons 2006 fireblade a much lighter bike. You must be sure there is NO air in the brakes at all. It may be overkill it is fantastic to have such super brakes plus it can be returned to stock quite easily. Will try and post photos for you later but those calculations of yours are spot on. Cheers Foxyj.

    ReplyDelete
  2. Deep thinkerDecember 17, 2009 at 8:06 PM

    since the single piston calipers are so large, you overshot the preferred ratio and got into the 35:1 range. you'll need to increase the size of your MC. your brake lever is likely too squishy.

    ReplyDelete
  3. UnknownDecember 18, 2009 at 3:03 PM

    Hi it does have the 5/8inch master cylinder and it does not feel squishy at all You just make sure there is no excess play in the lever travel and no air in the system and it woks perfect. Had it on the bike for 18 months now and its super. Will post up some pics shortly. cheers Foxyj.

    ReplyDelete
  4. binoFebruary 11, 2013 at 8:46 PM

    hi i have a 81 cb750c with single piston caliper on both sides . one of the calipers started to leak and i took apart and the piston has little craters in it. ive been looking but can only get at dealer . i ran into your blog and thought this be great. now my question is what caliper to use vf500. if so what year. any other pointer would be great thanks for this and your hel

    ReplyDelete
    Replies
    1. Deep thinkerJuly 24, 2013 at 1:36 PM

      Sorry for the late reply. If you haven't figured it out yet, z1enterprises and a few ebay vendors do have replacement brake pistons (last time I looked).

      As long as you search for the vf500 calipers on ebay, I don't think there were many years of that model and they should all be the same, however I'm a little removed from that subject matter and cannot remember if there's anything to warn you about.

      Delete

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