77 KZ650 master cylinder rebuild kit

    Just received my rebuild kit from brakecrafters for my front master cylinder. There is a part I’m not sure about included in the kit. It is what I would call a fluid damper. It is the first piece that goes in the bore. It sits in the big end of the spring. The question is when I disassembled it that part wasn’t in there and it’s also not shown in the diagram in the shop manual  so I’m assuming I don’t use it. Any input would be appreciated.

Looks like part #20? Primary cup?

No its not the pimary cup. the primary cup is rubber and pushes fluid in the bore. The piece in the picture is metal and has holes in it  and goes into the bore first if it is used.



 

Email Brakecrafters the photos you've shown here and they'll be able to answer.

Is it a "seat" for the spring to stop it chewing into the alloy ?

After further investigation I see that this part is a check valve. Basically it lets fluid go in one direction and not the other. But I’m still not sure if I should put it in. After looking at other parts diagrams of master cylinders of similar year motorcycles some have the check valve and some don’t…

I don’t believe it had one from the factory. I bought the bike from the original owner and it doesn’t appear to have been wrenched on much.

It's not a check valve or the fluid wouldn't be able to return to the reservoir, notice it has a bunch of holes in it. I'm with what Wookie said, it also might help keep the spring centered in the bore.
I really don't know what it's purpose is but I would use it if I were you. I think it would work fine without it but it's not going to hurt anything to use it, the round end fits inside the spring.

The nearest I could figure out from reading was it assists when bleeding the brakes 

It is most definitely a check valve. They are used on the 900’s and 1000 in both single and dual disc. Single disc part number is 43017-001. In the dual version they are part number 43017-002. Now I can’t advise if they are needed in your application. I agree as mentioned, call Brake Crafters. 

EDIT, looking at more parts breakdowns your bike and the 1000 A1-2 use the same piston (43072-001) and spring (43018-001). My gues would be Brake Crafters knows this and stocks one kit for both (and more I’m sure) applications. I’d say using it could be a choice but again I’d make the call. 

EDIT #2, here’s another twist. Both M/C use the same OEM repair kit (43073-001) on the 650 fishe they don’t show the check. On the A1-2 the check is shown. 

 

That's really interesting, when I first looked at the pic it appeared to have a hole in the centre but it looks like a rivet/stud etc. I guess it controls the release rate after the pressure is applied. Happy to be corrected, thanks Matt