Jim Grant's Tech Tips
70
Chevy Chevelle, Brake Problems
Q: The brake calipers on my '70 Chevelle will not let the rotors turn
by hand. I have installed a new master cylinder, new calipers, new brakelines
and new pads. When I say "Turn by hand," I mean with the tire mounted.
it takes all I've got to move it. There must be something wrong because
pads don't last very long 4000 miles or less?
A: The
reason the front brakes are staying on is due to residual pressure in
the brake system. Of course the question is why is this? If both disc
brakes are dragging then the problem is likely with the master cylinder,
even though it's new.
There
are 3 possibilities. 1. Check the push rod from the brake pedal to the
master cylinder. Some push rods are adjustable and the adjustment can
change. If the adjustment is too tight the push rod can actually cause
the brakes to drag in the manner you describe. The fix would be readjusting
the push rod or, if not adjustable, adding shims to provide the correct
clearance. 2. There are check valves used in master cylinders called residual
check valves. The residual check valve is used "only" on drum type brakes.
If a residual check valve is used on disc type brakes it will cause the
brakes to drag. Your vehicle's brake system requires a residual check
valve for the rear brake (if drum type) and no check valve for the disc
brakes. If the master cylinder has a check valve in each port then it's
the wrong master cylinder for your vehicle. 3. Human factor, be sure that
the rear brake line didn't get crossed some how with the front.
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