mitchp wrote:what is the function of the rubber/plastic valve assembly shown on the left of ChuckW's photo? I stripped an old spare master cylinder last week, but the rubber valve was in such poor condition that I couldn't work out what it does or how it works.
Quote from the Triumph T140 manual: "The purpose of the check valve.....is to prevent the re-entry into the master cylinder of fluid pumped into the line during the "bleeding" operation, thus ensuring a fresh charge of fluid at each stroke of the lever"
I believe it also serves a second purpose (again, quoting from the T140 man.)
"On releasing the brake lever the return spring thrust[s] the piston back faster than the fluid is able to return from the wheel cylinders."
(= due to the valve)
"This creates a partial vacuum in the cylinder which causes fluid to be drawn past the lip of the primary seal from the main reservoir via the main feed port and the small feed holes in the head of the piston."
So it also helps to keep the cylinder fully charged with fluid,-ready for the next brake application.