Tech Talk

How To: Diagnose and Fix Brake Drag

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Whether you are installing new brakes on your classic car or working on an existing system, brake drag can be a frustrating and challenging thing to diagnose and locate. If there is a good thing, the problem is usually a pretty simple repair once you find the culprit.

Brake drag is typically a function of mechanical issues or hydraulics.  Referring to the check-lists below will help you identify and repair your brake system’s brake drag woes so you can get back out on the road.

Mechanical Brake Drag

  1. Master Cylinder Push Rod Length: On manual brake systems (but also on a power brake system), the pedal needs a small amount of free play at the top of the travel. This guarantees that the pushrod is not keeping any pressure on the master cylinder. If there is a small pre-load, this will keep a small amount of pressure in the system causing the brake drag.
  2. Brake Booster Push Rod Length: Similar to the above scenario, the pin between the brake booster and the master cylinder can be too long. This will not allow the master cylinder to release completely. To correct this problem, it is necessary to determine the distance between the push rod and the piston. This needs to be approximately .020”. To aid in measuring this, Master Power offers a Push Rod Depth Gauge. It is p/n: AC2003 and sells for $25.00.
  3. Caliper Piston: Just like with the master cylinder not releasing causing the brake drag, a caliper not releasing and staying applied can do the same thing. If only one position is dragging, this could be the case.
  4. Misalignment of Caliper: If the caliper is not properly aligned with the rotor, a drag can occur. This is usually caused by a bent caliper mounting bracket or severely warped rotors and pads. To correct, visually inspect the alignment between the caliper and rotor. If the bracket is bent, replace as necessary. If the rotors are warped, you will experience a drag while turning the rotor followed by the rotor turning easy. This situation will then repeat for each rotor rotation. To correct this, replacement of the rotors and pads will be necessary.
  5. Incorrect Wheel Bearing Installation: This situation could come across as a brake drag but is a drag due to wheel bearings being seized due to a lack of grease or over tightening during installation. The easiest way to diagnose this is to remove the caliper from the rotor and turn the rotor. If the drag still is present, then the problem is internal with the bearings. At this point, replace as necessary and reinstall to the proper torque value and pre-load.
AC2003 - Push Rod Depth Gauge

AC2003 – Push Rod Depth Gauge

 

Hydraulic Brake Drag

  1.  Heat: Heat can cause brake drag in several ways but the most common scenario  is due to brake fluid reaching and exceeding its boiling point.  Verify there is adequate clearance between brake lines and exhaust and depending on the location of the master cylinder or booster/master, make sure there is ample room between those components and the engine or exhaust. To correct, shield the components in the system as necessary and replace the brake fluid with a high quality brake fluid such as our Pentosin Brake Fluid.
  2. Collapsed Flex Hose: There are a minimum of three brake hoses on the vehicle, and these lines are often overlooked when freshening up a brake system. The internal wall in the hose can come apart or wear out allowing the fluid to not release keeping pressure on the system. Just like coolant hoses, brake hoses should be inspected for signs of wear on a regular basis.
  3. Master Cylinder Piston: While similar to a mechanical brake drag scenario mentioned above, the piston inside the master cylinder may not be releasing. This situation is usually remedied by replacing the master cylinder.
  4. Master Cylinder Overfull: We see a lot of brake drag problems caused by a master cylinder that is too full of fluid. If the system is overfull, there may not be enough room in the reservoirs for the fluid to return. In most master cylinders, it is necessary to have the fluid level approximately ½” from the top. This will allow the fluid to return and leave the proper air space in the master cylinder.
  5. Metering Valve: In a brake system with disc front and drum rear, there is a valve in the system known as a metering valve or hold-off valve. This valve basically allows the rear drums to apply first in the system. It does this through a small sliding valve operating on spring pressure. If this valve is sticking or has become weak, it may be keeping pressure applied to the system. If this is the case, replace the valve as necessary.
ACJ1703 – Pentosin Brake Fluid

ACJ1703 – Pentosin Brake Fluid

 

While the above scenarios may not be the only things that can cause a brake drag in a system, they are the ones that need to be addressed first as the basics. Usually one of the above will correct most brake systems.  If you have additional questions about brake drag, or need to purchase replacement parts, visit our newly remodeled e-commerce enabled website at www.mpbrakes.com or call us at 1-800-397-2076.