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Tech Talk with Master Power Brakes

How To: Diagnose and Fix Brake Drag

Posted by toby on Jun 5, 2017 4:20:00 PM

Whether you’re 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. On a positive note, the problem is usually a pretty simple repair once you find the culprit.

What Is Brake Drag?

Brake drag happens when the brake pads or shoes don’t release completely when the brake pedal is released, and is typically a result of mechanical issues or hydraulics. It is highly recommended to fix the dragging brakes as soon as possible, because it can lead to overheated parts and potential damage to the braking system, or worse yet, an accident.  

Referring to the checklists below will help you identify and repair your brake system’s brake drag woes so you can get back out on the road.


 brake drags, how to diagnose brake drags 

 

 

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Potential Mechanical Issues Contributing to Brake Drag

 

Master Cylinder Push Rod Length

01 On manual brake systems, 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 brakes to drag.

 

Brake Booster Push Rod Length:

02 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 and for visual assistance, refer to Got A Soft Pedal? Don't Forget The Booster Pin to Master Cylinder Gap Master Power offers a handy Push Rod Depth Gauge Tool that every good mechanic should have in their toolbox.

 

Caliper Piston

03 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.

 

Misalignment of Caliper

04 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.

 

Incorrect Wheel Bearing Installation

05 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.

 

 
 
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AC2003
Push Rod Depth Gauge
 
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 Potential Hydraulic Issues Contributing to Brake Drag

 

Heat

01 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.

 

Collapsed Flex Hose

02 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.

 

Master Cylinder Piston

03 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.

 

Master Cylinder Overfull

04 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.

 

Metering Valve

05 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.

  

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ACJ1703
Pentosin Brake Fluid

Do You Have Additional Questions?  We’d Love to Answer Them!

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. 

Still got questions? Give us a call! 1-888-249-9425

 

Read more about fixing a soft pedal:
Got A Soft Pedal? Don't Forget The Booster Pin to Master Cylinder Gap

 

Topics: brag drag, brake hose, classic cars, how to, How To Series, hyfraulic brake drag, master cylinder, mechanical brake drag, metering valve, repair

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