It makes your pads drag--really.
A residual pressure valve temporarily maintains some pressure on the calipers
after you lift your foot. When you step on your brakes the pistons inside the
calipers press the pads against the disk, These pistons are free to slide
forward past the seals as the pads wear, which is why there is no adjustment
for disk brakes--they self adjust as the pads wear. The pistons are pulled
back away from the disk by the spring tension of the neoprene seals.
Essentially the seal sticks to the piston a bit, and when you release the
pedal the seal lips pull the piston back. Problem is that the lips may pull it
too far back, or the disk might have a little excess runout, so the piston
gets pulled back too far, and you have to pump the brakes to get the pads back
in contact with the disk.
A residual pressure valve maintains some of the pressure against the piston,
preventing the piston from fully retracting or getting pushed back by disk
runout. They hold about ten or fifteen pounds of pressure in the line.
Eventually the pressure leaks away, but while you're driving that pressure is
pretty much always applied, so your pads drag a little bit. I know it seems
like that would be a bad thing, but it's not. The small amount of braking
force applied keeps the pads slightly warm, causes no appreciable wear, and
off-gasses the material on the surface--all good stuff. The only time isn't
not so good is when you screw up and get fifteen pounds of line pressure
applied to a drum brake, which is adjusted close to the drum to begin with,
and winds up heating the drums and causing fade, or you have a residual
pressure valve in a clutch M/C, which keeps pressure applied to the throwout
bearing, which toasts the bearing and can ruin your crankshaft thrust washers.
Five pounds residual pressure (or none) is about right for a drum brake.
-----Original Message-----
From: Richard Taylor [mailto:tarch@bellsouth.net]
Sent: Fri 11/2/2007 12:46 PM
To: Bill Babcock; WEmery7451@aol.com; markvaden@gmail.com; fot@autox.team.net
Subject: RE: [Fot] dual circuit brakes on a tr4
Bill,
For those of us who don't know a residual valve from Shinola, what's it do?
Although I've been told it is normal, I have never been able to get the
front wheels of my TR-4 to rotate absolutely freely (while off the ground)
without some pad drag.
Richard
atlanta
It's not a balance vbalve, it's a residual pressure valve.
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