Master Cylinder return

[fvincent]

Hi Gang. Last fall I found a brand new front master still in the OEM box. Swapped it in. Bled both lines. Solid feel till yesterday. The brake lever almost went back to the grip on a routine stop. I pumped it once, and it felt fine. Went directly to my friend Pats house, (yamaha mech.) and found no leaks, no loss of fluid no stuck caliper, and no stuck piston. The front wheel turns freely. However I now have about 1/4" of free play in the lever. Its not the lever adjust, as the stop nut has not moved. Its almost as though the piston is not coming completely out of the master cylinder, but every thing works. Its a very weird feeling. Knowing a bit about physics, I cant think of a scenario of how this could be possible. As I said, no drag on front wheel & pads look great. Help?

Floyd
'80G

[DiverRay]

I would try to change the brake fluid first. Are you running the rubber OEM brake hose, or a SS aftermarket? The OEM CAN put fine rubber bits into the fluid, and cause trouble. This happens as the hose gets old, but ALL the OEM hoses are old now!
Just my $0.02

[egsols]

If it comes back after you pump it, there must be air in the system. After a MC replace it can take a while to get rid of all of the air, sometimes it hides ride inside the banjo at the MC and sometimes at the splitter on the forks, or both.

One trick I got off this site that helped me last year, after replacing the MC, was to pump the lever until rock hard and then hold to the bar with a clamp or bungee and let sit over night.

[HobbyMan]

Just because it is NOS, doesn't mean that the rubber inside is still good. Maby just bleed the brakes again. If that doesn't work, It sounds like you need a rebuild kit and maby some brake lines.

[FJ111200]

I agree partly with HobbyMan on this.
Sometimes the piston seals in the master cyclinder can go, and when pumping the lever the fluid just goes round through the return hole in the cyclinder, causing the said problem.

[TADracer]

As a matter of safety whatever you do, find the cause before you need to rely on your fronts for an emergency stop.

[fvincent]

Master Cylinder

Thanks people. I did buy a rebuild kit just before I found the master cylinder. 'Looks like now is the time to use it

Floyd

[79XS11F]

The same thing happened to me on my front MC and when I took the lever off I discovered that the little spring was not in it recess in the perch. It was binding on the MD piston and preventing it's full return to the neutral position. Check it out as a possibility.
Rob