bore size for rear master cylinder

need help ;  I posted a question about bore size on rear master cylinder, about a month ago and I got no replies, not sure if I did something wrong posting it . if anyone could help me I would appreciate it I am working on a 1980 kz1000G.

The bore size is 1/2 inch AFAIK, should be cast in the rear MC body......

 the bore is 5/8 but I purchased a new master cylinder for a newer bike . it mounted up alright but it has a smaller bore now I'm having trouble getting it to pump up i have bled the system several times my question was would the smaller bore and smaller reservoir cause it not to pump up

rlcarter wrote:

 the bore is 5/8 but I purchased a new master cylinder for a newer bike . it mounted up alright but it has a smaller bore now I'm having trouble getting it to pump up i have bled the system several times my question was would the smaller bore and smaller reservoir cause it not to pump up

Wow!  5/8 is huge for a rear master.  The J series 1000's uses a 14mm master.

rlcarter wrote:

 the bore is 5/8 but I purchased a new master cylinder for a newer bike . it mounted up alright but it has a smaller bore now I'm having trouble getting it to pump up i have bled the system several times my question was would the smaller bore and smaller reservoir cause it not to pump up

Not sure what you mean by "pump up", are you saying that the system is bled but doesn't build pressure when pushing the pedal ? if you push the pedal several times quickly does it build pressure ? it is possible the if the master/calliper hydraulic ratio is way off that a single push of the pedal doesn't displace enough fluid to apply pressure in the calliper !
A "braided" line may help as there will be less "expansion" in the hose than the stock rubber item which steals pressure from the calliper.
 

the new master that I put on is a smaller bore  I suppose the ratio could be off but I have tried pushing the pedal several times quickly , still no pressure .I took the line loose at the caliper and pushed the pedal, it is pushing fluid.i do have an after market line on it which I had when the original 5/8 master was on and it worked.i am gonna double check for leaks  but in the mean time try to find a rebuild kit for the 5/8 master  thanks for replying I will see what I find and let you know
 

rlcarter wrote:

the new master that I put on is a smaller bore  I suppose the ratio could be off but I have tried pushing the pedal several times quickly , still no pressure .I took the line loose at the caliper and pushed the pedal, it is pushing fluid.i do have an after market line on it which I had when the original 5/8 master was on and it worked.i am gonna double check for leaks  but in the mean time try to find a rebuild kit for the 5/8 master  thanks for replying I will see what I find and let you know

 

Is the replacement cylinder "after market" - you could try disconnecting the line from the master and inserting a 10x1.25 bolt then see if the master actually produces any pressure (do it carefully or you could flip the seals) worth a try before you keep throwing money at it ?

yes it is after market  I will try and block it off .thanks wookie