Motorcycle Brakes Won’t Hold Pressure? Here’s Why (Solved)

The brakes we use are fundamental to our safety while out riding on our bikes, so it’s alarming (and a bit confusing) when they won’t hold pressure.

Fortunately for the reader, yours truly has spent the last week tearing apart the entire braking system on a finicky motorcycle.

So let’s go ahead and dig into diagnosing and fixing some of those pesky, frustrating brake pressure issues!

Why Won’t My Motorcycle Brakes Hold Pressure?

To understand why a braking system will not pressurize, we must first understand how the brake system works. Although diagnosing a problem within a hydraulic system might sound like a daunting task, it’s quite simple!

Braking systems rely on hydraulic fluid to create the pressure needed for braking.

Squeezing the hand brake lever, or applying pressure to the rear brake pedal, causes the brake fluid to pressurize the caliper piston and slide arm. This works because of the relationship between the master cylinder and caliper. 

The master cylinder is the internal mechanism that pumps pressure into the line when you apply pressure to the right hand or foot. The vacuum line is where the fluid is trapped between the master cylinder and caliper.

The caliper is also known as the slave cylinder. It receives the pressurized hydraulic fluid, which, in turn, pushes the brake pads against the rotor.

This causes the friction you need for stopping power. The pads are pushed into the rotor by pressurized pistons in the caliper. 

Squeeze the brakes and you pressurize the system-let off the brakes and the braking system depressurizes, retracting the pistons and brake pads.

Properly pressurized brake systems should respond to the slightest touch of the hand or foot. If the controls feel spongy or you are taking far too long to stop, there isn’t enough hydraulic pressure to move the caliper’s pistons or slide arms.

One thing to consider before tearing into brake line diagnosis-check your brake pads! Brake pads wear down over time by rubbing against the rotor. A brake system is usually pressurized when the pads are thicker and essentially further apart. 

If you are feeling your brake system depressurizing over time, it’s quite possible you just need to replace the pads and can stay away from a messy, time-sucking brake diagnosis!

If Your Brakes Won’t Hold Pressure, Here’s Why:

Brake lines can be filled with bad fluid, air, water, corrosion, and debris.

Vacuum leaks in a faulty line, banjo bolts, bleeder valves, or vacuum seal will also depressurize the braking system, rendering it ineffective.

The following symptoms can lead to the proper diagnosis and solution, so let’s go for a ride (only figuratively, don’t ride a motorcycle without brakes)!

Old Fluid

Brake fluid is the hydraulic liquid that is used to pressurize the brake line. Fluid that is 1-2 years old should be replaced with fresh, clean fluid.

This is because brake fluid is hygroscopic. In essence, brake fluid that is exposed to air will attract water molecules, causing rust and corrosion. 

Brake fluid comes in several varieties-DOT 3, 4, 5, and 5.1. Almost all brake fluid reservoirs will have the correct fluid type for your bike printed on the cap. This is the only fluid you should use on your bike! 

DOT 3, 4, and 5.1 are all light yellow, glycol-based brake fluids that can be mixed, but make sure to use the fluid rated for your bike. 

DOT 5 brake fluid is a purple, silicone-based fluid that cannot be mixed with any other fluid. Doing so can severely damage your brake lines and is both costly and time-consuming to fix.

Old, dirty brake fluid is a brownish color, as opposed to clear, clean fluid. This is because, over time, water and air have corroded the inside of your brake system, adding rust into the mix.

To check the quality of your brake fluid, simply open the brake fluid reservoir on the right side of your handlebars. Be sure to clean the lid before you open the reservoir. We don’t want any contaminants like dirt, dust, or debris to enter the brake line at any time.

Having opened the reservoir, you will see the brake fluid. If it is clear and viscous, then move on to another symptom. If it is dark or even sludgy, it’s time to bleed new fluid into the line, removing the old fluid and any contaminants with it.

Bleeding the brakes is much less wild than it sounds! On your brake caliper will be a small bleeder valve.

This valve is a threaded screw with a hole through the center. It will have a small rubber cap on it to keep any contaminants out of the system as it is being bled. 

Simple Instructions for Bleeding Your Brakes

• Cover your gas tank and any other painted areas with rags or shop clothes. Brake fluid is a corrosive liquid that can eat through paint.

• Carefully open the fluid reservoir. Clean the reservoir lid before opening it to keep out any contaminants. If the reservoir hasn’t been opened in a while, the rubber gasket tends to form a vacuum seal. When opening the reservoir, the gasket can pop open and splash brake fluid around. It is imperative that you keep this reservoir topped off with new fluid as the old fluid slowly drains out. If the reservoir empties, this will introduce air into your brake lines and this process will take a lot longer.

• Locate the bleeder screw. Remove the small rubber nipple from the head of the bleeder screw.

• Attach plastic tubing to the bleeder screw head. Place the other end of the tubing in a spare plastic bottle to collect the old fluid.

• Pump the brakes. About three gentle pumps should do. Keep the brake depressed for the next step. 

• Crack open the bleeder valve with a crescent wrench ¼-½ turn. This will begin to slowly drain the pressurized fluid through the plastic tubing down into the container.

• Now close the bleeder valve again.

• Pump the brakes, crack the valve, and close the valve. Keep an eye on the fluid reservoir, topping it off with new brake fluid all the time. Once the fresh, clear fluid is coming out of the brake line without any air bubbles, the system has been bled and is ready to be tested.

Most pressurization issues in your brake line will be solved by bleeding your brakes, so it’s an important process to know how to do. And really getting into the brake system will help your understanding of the braking process.

Like anyone who has ever bled brake systems, we know that it can be incredibly time-consuming. Consider investing in a vacuum pump brake bleeder available at most auto parts stores for $40-$60.

It can seriously reduce the time it takes to do this process. After all, time spent bleeding is time that could be spent riding!

Related: Motorcycle Brake Caliper Won’t Release: 5 Reasons (Solved)

Air in the Brake Line

Perhaps the last time your brakes were bled, air got into the line. This will keep them from holding the essential pressure needed for the braking system to function properly.

This happens when the reservoir goes dry and the master cylinder pumps air bubbles into the line. That is why it’s incredibly important to keep the reservoir topped off at all times.

Another reason your brakes won’t hold pressure is a vacuum leak.

Corroded rubber on the brake line or bad seals on either end of the vacuum line can slowly cause an air buildup in the hydraulic fluid.

This allows air and water molecules to penetrate the line.

Inspect your brake hose for any cracking or holes in the rubber. If it is too old and corroded, it will need to be replaced.

Next, check the seals at either end of the brake line. If there is any fluid seepage, you will need to replace the washers on either side of the banjo bolt that holds the vacuum line in place next to the caliper and the master cylinder. 

Once the seepage issue has been rectified with new washers, the line will need to be bled again to pressurize the brake line.

Wrong Brake Fluid

We cannot stress how important it is to use the correct fluid when bleeding or refilling your brake lines. 

Glycol-based DOT 3, 4, and 5.1 must never mix with the silicone-based DOT 5 fluid. They will mix poorly, causing a jelly-like substance that will gum up your entire brake line and you will have to rebuild the brake line from scratch.

Carefully follow the specifications for your motorcycle on which fluid to use. 

If you aren’t sure what kind of brake fluid is in your bike, there is a water test you can do to find out.

Simply remove a bit of fluid from the reservoir with a small cap or extraction syringe and put it in a bit of water. Put a lid on the container and shake. 

DOT 3, 4, and 5.1 will eventually mix with the water because it is hygroscopic. DOT 5 will never mix with the water because it is silicone-based and water repellant.

Related: Motorcycle Brakes Won’t Bleed 4 Reasons (Explained)

Blockage in Master Cylinder Port

In the master cylinder are two small ports. One allows fluid from the reservoir to enter the brake line. The return port allows it to flow back through the cylinder to the reservoir when the pressure is released.

If any of the aforementioned issues have corroded the inside of your brake line, or if any foreign debris has been introduced, it can block the small ports in your master cylinder.

To fix this issue, carefully open the brake fluid reservoir. The ports between the reservoir and master cylinder are visible from here.

Using something like a needle, safety pin, or a pic, carefully unclog any debris from the blocked port. It’s usually the smaller return port.

If you have debris or corrosion blocking the ports, bleed new fluid into your brake lines, removing any older fluid that carries the debris or corrosion.

Bad Master Cylinder 

So you’ve bled your brakes thoroughly, but the line still won’t pressurize. This can be due to a faulty master cylinder.

If the metal rod and spring are broken, or the rubberized seals at the end of the cylinder are bad, it won’t push fluid into the hydraulic line. 

The master cylinder is located under, or next to, the fluid reservoir. You will need to remove the brake lever to get to the inner workings of the master cylinder.

Remove the brake lever to expose the exterior push rod. Remove this pushrod as well as the rubber boot that it sits inside. Be careful when removing the rubber boot because the cylinder and spring inside can pop out. 

Check rubber boot, cylinder, inside rubber gasket, and spring for signs of wear, broken parts, or cracking. If anything is amiss, you will need to rebuild the master cylinder.

Related: How To Tell If Motorcycles Have Bad Brakes (Explained)

Sources

How Do Motorcycle Brakes Work | FamilyHandyman.com