Stuck Brake? No Brake? Problems After Replacing Rubber Seals or Repair Kit on the Master Cylinder

Updated on January 24, 2019

The parts that easily get worn out in a brake master cylinder assembly are the rubber seals or cups. They also have the lowest cost of replacement. The piston sub-assembly comes next in the wear-and-tear category and costs a little bit more to replace. The cylinder rarely gets worn out and takes some time should it ever wear out, but it has the highest cost of replacement because the entire master cylinder assembly would have to be replaced. You cannot buy a cylinder only.

Despite all this, some mechanics opt to replace the entire master cylinder assembly because only replacing the seals or repair kit creates more problems than what had originally been encountered. The new problems could be loss of brake and stuck brake, while the original problem was only the sinking of the brake pedal when it was stepped on (a symptom of worn out seals).

These two problems, loss of brake and stuck brake, share a common cause: a blocked compensating port.

What Is a Compensating Port?

First, take a look at Fig. 1. This is the brake master cylinder assembly I’m using for illustration. All master cylinders of this type operate in the same principle, regardless of brand or make. Inside this cylinder are stuffs shown in Fig. 2.

The compensating ports are those small holes beside the inlet ports.

Figure 1 Brake Master Cylinder Assembly
Figure 1 Brake Master Cylinder Assembly

In a little while, I will be telling you how the compensating ports can be blocked, how this can be avoided, and how to detect a blocked port. For now, familiarize yourself with the different parts of the master cylinder. See Fig. 3, 4 and 5.

Piston Assembly Removed

Click thumbnail to view full-size

What Can Block or Clog the Compensating Ports?

  1. The wrong types of seals were used.
  2. The wrong stroke adjustment was used.
  3. The wrong piston was used.

1. The Wrong Type of Seals Are Used

See Fig 6.

Solution: Use rubber seals that are similar to the original ones.

2. Wrong Stroke Adjustment.

See Fig 7.

Replacing the front seal of the primary piston requires that the stroke adjuster screw be unscrewed and removed. When reassembling the primary piston assembly, the correct stroke may not be obtained at once.

Solution: Adjust the stroke adjuster just enough to clear the compensating ports. Turn the adjuster screw clockwise to shorten the stroke and couter-clockwise to extend the stroke as shown in Fig. 5.

3. The Wrong Pistons Are Used

See Fig 8.

Solution: Use pistons that are similar to the original. Or, if you wish, you can do a McGyver and grind or machine cut the butt of the pistons to a thickness that is just enough to clear the compensating ports.

How Do We Know If the Compensating Ports Are Blocked?

How Do You Know If the Compensating Ports Are Blocked?

To determine whether or not the ports are blocked, you can do any of the following after the seals or repair kit have been installed and the entire cylinder assembly has been assembled:

  1. Using a tube with a fitting on one end, install the fitting on one output port and blow into the tube. If you cannot blow into the tube, the compensating port is blocked.
  2. Put a little fluid in the reservoir and see if fluid comes out of the output ports. If no fluid is coming out, the compensating ports are blocked.

Now you know what compensating ports are, how they can be blocked, what to do when the compensating ports are blocked, and how to detect if the compensating ports are blocked. You can now carry out a successful repair job. However, if you would like to know why a blocked compensating port can result in a loss of brake or a stuck brake, please read on.

How Does the Brake System Work?

Principles of Operation

Fig. 9A shows a simplified brake system diagram at normal condition. Here the pistons are fully retracted, the system is filled with fluid, and the caliper is open, setting the brake disc free.

When the pedal is stepped on (Fig. 9B), the pistons move to the left, pushing more fluid into the caliper cylinder. This causes the brake pads to come into hard contact with the brake disc.

When the pedal is released (Fig. 9C), the pistons move to the right. This causes a partial vacuum at the pressure chamber, causing additional amounts of fluid to be sucked into the pressure chamber via the small holes on the piston and through the back of the rubber seal.

By the time the pistons have fully retracted (Fig. 9D), the compensating ports are opened, allowing the fluid to return to the reservoir as the caliper piston retracts. This sets the brake disc free.

How Does a Blocked Compensating Port Cause a Stuck Brake? (Fig. 10)

As illustrated above (Fig. 9C), additional fluid is sucked into the pressure chamber as the brake pedal is released. However, this amount of fluid cannot return to the reservoir if the compensating ports are blocked (Fig. 10). This prevents the calliper piston from retracting fully.

Repeated application of the brake causes fluid pressure to build up in the pressure chamber even when the brake pedal has been fully released. As a result, the brake pads remain in hard contact with the brake disc, thus, a stuck brake.

How Does a Blocked Compensating Port Cause a Loss of Brake? (Fig. 11)

Note: Loss of brake due to blocked compensating port can occur only if the fluid lines and wheel cylinders are empty, as in the case when the rubber seals of the wheel cylinders are replaced.

Fig. 11A shows a simplified brake system in a fully retracted sate. Since the compensating ports are blocked, no fluid can enter into the pressure chamber and into the wheel cylinder.

Fig . 11B shows the brake system when the pedal is stepped on. In this case, the pistons move to the left, compressing the air in the pressure chamber and in the wheel cylinder. Since air is compressible, the pressure in the wheel cylinder won’t be enough to push the piston out, leaving the calliper open. Hence, no brake.

The usual thing to do in this case is to purge the system to remove the air by pumping the brake pedal a couple of times, then holding the pedal down, and then opening the bleeding plug at the remotest wheel cylinder. This is repeated until the brake pedal is hard and at a considerable distance from the floor.

This won’t work if the compensating port is blocked. The reason is this: when the pedal is pushed down, the air in the pressure chamber and in the wheel cylinder is compressed (unlike fluid, air is compressible). When the pedal is released, the compressed air will just return to its original uncompressed state. No partial vacuum is created, and hence, no fluid is sucked into the pressure chamber no matter how many times the pedal is pumped on.

As noted above, loss of brake due to a blocked compensating port, which can occur only if the system is empty, as in the case when the rubber seals of the wheel cylinder are replaced. If only the master cylinder was removed for repair, the fluid lines and the wheel cylinders are still filled with fluid. In this case, purging the system can be successful in having a hard brake, but we would then be confronted with the other problem of stuck brake as explained above (Fig. 10).

Replacing the entire brake master cylinder assembly may be the easiest way to solve master cylinder problems especially if rubber seals and repair kits are not available. However, if we want to save on the cost of repair and if rubber seals or repair kits are readily available, we may opt to replace these parts only. We just have to be sure that the compensating ports are clear of obstruction.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


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    • profile image


      2 years ago

      Y get the front brake stuck when was welder near of brake lines.

    • profile image

      michael nyda 

      3 years ago

      Hi i had replaced all my rotors and pads.The performance brakes are better than normal brakes,but for some reason the front brakes rub really hard when pushed.Everything is new except the calipers.I hope you can give me the answer to the problem.


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