Do You Bleed Brakes With the Cap On or Off?

Having properly bled brakes is crucial for safe and effective braking. But one key question many DIYers struggle with is whether to leave the cap on or off the brake fluid reservoir during the bleeding process. Though it may seem trivial, this simple choice can make or break your brake bleeding endeavors.

Here’s a quick answer: When bleeding brakes, it is generally recommended to leave the brake fluid reservoir cap off in order to allow air bubbles to escape as fluid fills the brake lines. Keeping the cap off makes it easier to monitor and refill the fluid level as needed. However, some vehicles may specify bleeding the brakes with the cap on – always check your owner’s manual for the proper procedure.

The Importance of Bleeding Brakes

Before diving into the cap on vs cap off debate, it’s important to understand why bleeding brakes is necessary in the first place.

Brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. This causes the boiling point of the brake fluid to drop, leading to vapor lock which reduces braking power. Moisture in the lines can also cause corrosion and damage.

In addition, air bubbles in the brake lines compress under pressure instead of transmitting hydraulic force to the calipers and pads. This severely reduces stopping power.

Bleeding brakes removes air bubbles, moisture, and old fluid from the system. This maintains proper brake function and pedal feel.

The Case For Leaving the Cap Off

The most common advice is to leave the reservoir cap off during bleeding. Here’s why this technique is preferred:

  • Allows air to escape: With the cap off, air bubbles can freely rise up through the reservoir as brake fluid and pressure pushes them out of the lines. Capping the reservoir can trap air inside.
  • Lets you monitor fluid level: Keeping the cap off allows you to easily watch the fluid level and top it off as needed.
  • Aids in one-person bleeding: Having the cap off makes it easier to bleed solo, pumping the pedal while fluid fills back up from the reservoir.
  • Prevents fluid overflow: If the reservoir overfills with the cap off, excess fluid can spill out instead of being forced back into the brake lines.

For most vehicles, leaving the reservoir cap off during bleeding is recommended for smooth, mess-free results.

Reasons to Leave the Cap On

Though less common, there are some scenarios where bleeding with the cap on can be appropriate:

  • Prevents contamination: Leaving the cap on helps keep debris, dirt, and other contaminants out of the brake fluid reservoir during the process.
  • Avoids spills: Keeping the cap on ensures brake fluid won’t accidentally spill or splash out of the reservoir as the pedal is pumped.
  • Maintains pressure: Some bleeding procedures require building up hydraulic pressure in the system, which is easier to maintain with the cap sealed.
  • Simplifies bleeding: Topping off the reservoir is unnecessary with the cap on, since no fluid can escape. This can streamline bleeding.

For certain vehicles, capped bleeding may be specified by the manufacturer. Check your owner’s manual to be sure.

Step-By-Step Bleeding Instructions

Bleeding your brakes is straightforward if you follow a few key steps:

  1. Start with a fully charged battery to power the ABS pump during bleeding.
  2. Check and top off brake fluid using new, clean DOT 3 or 4 fluid.
  3. Inspect brake lines for damage or leaks. Tighten fittings as needed.
  4. Loosen bleeder screws on each caliper, but don’t remove them.
  5. Have an assistant pump the brakes. Hold pressure, then open each bleeder to release fluid and air bubbles.
  6. Tighten bleeders, then refill reservoir as needed. Repeat process until air stops escaping.
  7. Ensure pedal feels firm, not spongy. Top off fluid level and replace caps.
  8. Test brakes in a safe area before driving.

Always consult your vehicle repair manual for the proper bleeding procedure. Safety first!

Frequently Asked Questions About Brake Bleeding

Still have questions about getting the air out of your brake lines? These common FAQs can help:

How often should I bleed my brakes?

Brakes should be bled any time the pedal feels soft or spongy. Many mechanics recommend bleeding the system every 2-3 years as routine maintenance.

What are signs I may need to bleed the brakes?

Low or inconsistent pedal feel, need to pump brakes to build pressure, brakes feel “mushy”, grinding or squealing noises, and longer stopping distances can all indicate air in the lines.

Does all the old fluid get replaced when bleeding?

Just bleeding won’t completely flush the system – for that, a flush using a brake fluid exchanger may be required. But bleeding does remove old contaminated fluid and replaces a good portion of it with fresh brake fluid.

Can I drive after bleeding brakes myself?

It is usually safe to drive after bleeding brakes at home as long as you followed proper procedures, topped off the fluid, and the pedal feels firm and consistent. Always test brakes at low speeds first.

What type of brake fluid should I use?

DOT 3 and DOT 4 fluids are both common. Use the type recommended by your vehicle manufacturer. Never mix fluid types.

The Bottom Line

Properly bleeding your brakes is an essential maintenance task for safe, reliable braking performance. While leaving the reservoir cap off during bleeding is the most common technique, as it allows air to easily escape, some situations do call for keeping it capped. Check your vehicle repair manual for specifications. Monitor fluid level, bleed each brake line, and test pedal feel often during the process.

With the right procedures and proper precautions, you can effectively bleed your own brakes at home between professional servicing. Just be sure to properly dispose of old brake fluid and keep an eye on fluid levels to avoid any leaks or issues. Safe braking is well worth the effort of a thorough brake system bleed.

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