1977 750f rear master cylinder

[Petes750]

Hey all, in the process of rebuilding the rear MC (first timer) I am finding the plunger binds up against the cylinder wall when compressed and not allowing a return to resting position after assembled. Appears as if the tolerance is too close metal to metal. Do I need to hone this out and if so does someone have the correct specs? Or am I looking at this wrong. The inside appears pretty clean and proper but I am guessing it’s not…?

Anyone wanna set me straight? Thanks - Pete

 

[Petes750]

I think I got it after a little looking around here. Seems to be working

 

[hinksy]

I had this problem with my front master . I stripped and rebuilt my entire brake system , then shelved it away untill I had rebuilt the rest of the bike , 2 years . When I went to bleed the front master , it would stick in and not return . I tried all manner of repairs and cleaning but found out the seal had hardened due to it being dry and sitting for so long , or they were just shit seals from the getgo . Make sure you lube up the piston with brake fluid when installing , so wet not dry . Check your clearances as they could be bad , but unless it was hammered dry when it was used it should be ok . Could just be dry seals

 

[Petes750]

Thanks for the info.

So I did a wet install and at first the metal of the piston plunger was binding on the cylinder.
Ended up taking a light grit small dremel tool to the barrel and took off a pretty good amount of residual from the inside. After that, reinstalled wet and it plunged up and down without binding as it previously did. Gonna bleed them tomorrow and see what I got

 

[hinksy]

I ended up using emery paper on the piston shaft so the seal didnt stick out so far and that seemed to work . But I didnt trust it as I couldnt trust that the seal wouldnt fail at some crucial point in time , so I got a new seal kit and bingo it worked , aswell as the seals were soft

 

[Petes750]

Yep. That did it. She’s building and holding pressure, bleeding complete. Wrapped that up. Few more things to wrap up on this bastard child of a ride and she’s ready for the new owner -