Re: Brakes problem
The check valve is found right in the outlet of the master
cylinder. Look in the hole - if you can see the piston
inside it isn't there. Sometimes it's a spring loaded
ball in a brass keeper, or a rubber flapper.
The check valve keeps about 2-5 lbs pressure in the line
to the rear. Reason is the rear cups are tapered cups, if
no pressure is kept on the edge, the edge can let a tad of
air in. As you drive it keeps letting in a little more
air until you have enough bubble to cause trouble.
Discs use a square O ring, and the pads are always up against
the rotors. If you kept any pressure in that line - ie
check valve, they would constantly keep the brakes applied.
Rebuilders lately, especially the ones that supply the discount
stores like ZonedOut or Krappen take them out. If you adjust
the rear brakes excessivly tight it will work without the
valve -- and they get to sell you new shoes and drums long before
you should have needed them because they wear too fast too
Take the old valve out of an old MC.
And - the latest thing on shoes - they make the "lifetime"
linings - they make them out of super hard material so they
will last longer, but they don't stop very well.
They'll wear your drums out before anything - if they don't
kill you first.
Compare the expensive "lifetime" trash to the cheap stuff.
The cheap ones are much softer, and stop much better.
The valve/block that that has 4 lines on it - the differential
valve, is NOT the proportioning valve.
That one is used on dual master cylinders - ones where the
front to rear systems are isolated fluid wise. The intention
is in case of a massive master cylinder failure (low fluid etc,)
the little piston inside moves to the side because of the
pressure differential and lets fluid get to the end that
failed. It also usually has the switch to operate the
brake light to inform you there's a problem.
Sometimes that differential valve and the proportioning valve
are built into the same big block of brass, in that case it's
called a Combination Valve.
Many master cylinders that were intended to operate two types
of brakes, discs and drums, have different sized bores,
that gives the different pressures needed for the different
systems with the same pedal effort. Else you end up with
different braking efforts front to rear, creating a
"swapping ends time."