70 Chevelle - Brakes stay on
Reggie in Barnsley, Yorkshire.
Question
Dr. John, I imported a classic from the US last year and the problem I am having now is 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 brake lines and new pads. When I say "Turn by hand," I mean with the tyre 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?
Answer
Reggie, the reason the front brakes are staying on is due to residual pressure in the brake system. Now 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 somehow with the front when fitting to the m/cylinder.
You didn't mention whether you changed flex brake hoses, if they weren't I would change them as well especially if they are originals. These can colapse internally and cause a similar problem though it usually happens on one side or the other. There is also one fitted at the rear axle.
Good Luck!
Dr. John
Back
The Car Clinic
