1979 KZ650 SR rear master cylinder woes

Did you check the rest of the rear brake components, ie. the hose condition and if the rear caliper is in working condition. Hope your diagnosis is correct. I’d hate to see you spend money and time to “rig” a MC and then find out it wasn’t the problem. Good luck.

There is a super tiny hole in the master wall which must be open in order for fluid to reach the pumping chamber properly. Very good chance yours is plugged.
It's good practice to prime the master manually by filling the pumping chamber and testing the unit on the bench. Sometimes the lack of pumping action is due to a bubble blocking fluid flow filling the chamber.
Be sure to replace your brake line and rebuild the caliper at the same time. There is sure to be sludge in the system inside these parts which must be removed in order for the system to function properly long term.

Mikaw wrote: Did you check the rest of the rear brake components, ie. the hose condition and if the rear caliper is in working condition. Hope your diagnosis is correct. I’d hate to see you spend money and time to “rig” a MC and then find out it wasn’t the problem. Good luck.

You're right. I need to slow down and make sure everything else is correct before I botch up a Vstrom part on a KZ:) The weather is finally getting nice and this is my last issue (for now) so I'm getting too anxious. Thanks for all you input. Really appreciate it !

Nessism wrote: There is a super tiny hole in the master wall which must be open in order for fluid to reach the pumping chamber properly. Very good chance yours is plugged.
It's good practice to prime the master manually by filling the pumping chamber and testing the unit on the bench. Sometimes the lack of pumping action is due to a bubble blocking fluid flow filling the chamber.
Be sure to replace your brake line and rebuild the caliper at the same time. There is sure to be sludge in the system inside these parts which must be removed in order for the system to function properly long term.

Very good suggestion. I'm going to pull it off, replace the brake line, and bench test it before trying anything else. Appreciate the help !

doonie wrote:

Nessism wrote: There is a super tiny hole in the master wall which must be open in order for fluid to reach the pumping chamber properly. Very good chance yours is plugged.
It's good practice to prime the master manually by filling the pumping chamber and testing the unit on the bench. Sometimes the lack of pumping action is due to a bubble blocking fluid flow filling the chamber.
Be sure to replace your brake line and rebuild the caliper at the same time. There is sure to be sludge in the system inside these parts which must be removed in order for the system to function properly long term.

Very good suggestion. I'm going to pull it off, replace the brake line, and bench test it before trying anything else. Appreciate the help !

Make sure that tiny hole is open too.

I checked Partzilla and your bike uses the same rear caliper piston as used on several models of kz1000, up to 1980. I would asssume you’d be able to put a kz1000 rear MC on. Do you happen to know your MC bore diameter.

I think you made a good choice to fall back and regroup. Brakes are not something you want to “rig”.

See photos of relief hole Nessism is talking about.

so here's what i ended up doing as I was getting nowhere with the original master cylinder. I bought this cheap knock-off on Ebay. It bolted on directly (different bolts, no big deal) which simplified things. I had to modify the bottom end of the new MC but didn't do anything to the bike that would prevent me from putting the original back on. I simply extended the bottom of the plunger rod and then rotated the barrel bolt so I could thread up from the bottom. The reservoir bolted on very easily right next to the fuse box. Bled the brakes and all is good.
Just wanted to provide an update and thank everyone for all the great advice and taking the time to assist. Really appreciate the help !
Gary

Thanks for the update. Be safe

Just a note, if you move piston back and forth "dry" with no fluid when assembling it can pull piston seal out of position and lose pressure.
When you disassemble is the seal still in place ?
Small cleaning with scotchbrite should work for surface if not pitted.

Just to let you know, a kz 650 rear master Is the same extrution as a kz1000 ! Yes, you can bore it to 5/8 to use Z1 enterprise kz1000 kit.
That is pre- 1980. 1979 back