How to Drain a Brake Booster Properly – A Step-by-Step DIY Guide

Having problems with your brake booster? Noticing symptoms like a hard brake pedal, increased stopping distance, or brake fluid leaks? If so, draining the old brake fluid and giving your brake booster a flush is often the solution. While it may sound intimidating, draining a brake booster is actually a straightforward DIY task you can tackle in your own garage with some simple tools and supplies.

Here’s a quick step-by-step guide:

  1. Remove the brake booster – Disconnect the vacuum hose, separate the master cylinder, unbolt the booster from the firewall.
  2. Drain old brake fluid – Locate the drain valve/bleeder screw and open it to release contaminated fluid.
  3. Clean out the booster – Use solvents to dissolve debris and sediment inside the empty booster.
  4. Refill with fresh brake fluid – Close the drain, pour new DOT3/DOT4 fluid into reservoir.
  5. Reinstall brake booster – Reconnect the vacuum line and master cylinder.
  6. Bleed the brakes – Open bleeder valves to remove air bubbles from the system.

Why Drain Your Brake Booster?

Before jumping in, it helps to understand why draining the brake booster is necessary in the first place:

  • Over time, brake fluid absorbs moisture which leads to corrosion and decreased brake performance
  • Draining flushes out old contaminated fluid and debris
  • Putting in fresh fluid ensures optimum hydraulic pressure for safe, responsive braking
  • Failure to drain brake fluid regularly increases wear on brake parts

Not draining brake fluid is asking for trouble. But fortunately, giving your brake booster a flush takes just an hour or two for most vehicles.

Gather the Right Draining Supplies

You’ll need a few basic supplies before getting started:

  • Brake fluid – Use fresh DOT 3 or DOT 4 fluid. Check your vehicle specifications.
  • Wrenches & socket set – For removing bolts to access the booster.
  • Clean container – To collect old brake fluid as it drains out.
  • Funnel – For pouring in new brake fluid.
  • Rags, gloves & eye protection – For safety when handling solvents/fluids.

You likely already have versions of these supplies in your garage. Pick up anything missing beforehand so the job goes smoothly.

Step-by-Step Brake Booster Drain Instructions

With your supplies ready, follow these key steps to drain your brake booster properly:

Remove the Brake Booster

First, locate the brake booster, usually on the driver side firewall behind the master cylinder. You’ll need to detach it. Disconnect the vacuum hose running to the booster. Removing the retainer clip makes this easier. Separate and remove the master cylinder so the booster is fully accessible.

Be prepared for some leftover brake fluid to drain out. Unbolt the brake booster from the firewall. There are typically 2-4 bolts. Use wrenches/sockets to remove. Pull the booster unit out slightly from its housing to reach the fluid lines. Take care not to bend or damage any components.

Supplies NeededDetails
Brake fluidFresh DOT 3 or DOT 4 fluid
Wrenches & socket setFor removing firewall bolts
Clean containerTo collect used fluid
FunnelFor pouring in new fluid
Rags, gloves & eye protectionFor safe handling of solvents/fluids

Drain the Old Brake Fluid

With access to the brake booster and fluid lines, you can now drain it. Locate the drain valve/bleeder screw on the brake booster unit. There may also be a drain plug. Place your drain container underneath to catch old brake fluid. Wear gloves/eye protection. Open the bleeder screw or remove the drain plug. Allow all old contaminated fluid to drain out until the flow stops.

Expect at least a few ounces of old brake fluid to come out. Allow enough time for it to fully drain.

Clean Out the Booster

Before sealing everything back up, take a minute to clean out the brake booster. Use acetone or brake parts cleaner as a solvent to wipe down inside the empty booster cavity. This dissolves debris and brake dust. Flush out any remaining sediment by spraying brake cleaner into the bleeder valve as you open/close it. Double check old fluid, residue, or debris buildup is fully removed.

Refill Brake Fluid & Reinstall

You’re now ready to finish up. Close bleeder screw/drain plug once flushed to seal booster cavity. Refill with fresh DOT 3/DOT 4 brake fluid through the master cylinder reservoir. Reinstall brake booster & reconnect master cylinder plus vacuum line. Top off reservoir with more fresh fluid as needed.

Bleed the Brakes

The last step is bleeding the brakes to purge any air bubbles. Bleed each wheel cylinder one-by-one according to your vehicle repair manual. Refill reservoir with fresh fluid between bleeds to prevent air from re-entering. Test brake pedal feel. Bleed again if pedal is soft or spongy indicating trapped air. Replace lid on master cylinder reservoir once bleeding is complete.

And that’s it! With a freshly drained and refilled brake booster, your brake system hydraulic pressure and performance will be restored.

Post-Service Checks

Before hitting the road, check your work:

  • Verify master cylinder and booster are tightly secured with all fasteners tightened properly.
  • Ensure brake fluid reservoir is filled to the “Full” line.
  • Start engine and check for fluid leaks around the brake booster. Tighten fittings or replace damaged seals as needed.
  • Test brake pedal by pumping it multiple times. It should feel firm and steady, not spongy.
  • Take vehicle for a test drive in a safe area. Stopping distance should be reduced compared to before the brake service.
Draining StepDetails
Remove boosterDisconnect vacuum & lines. Unbolt from firewall
Drain old fluidOpen bleeder screw/drain plug to release fluid
Clean boosterFlush with acetone/brake cleaner solvent
Refill and reinstallClose bleeder, refill fluid, reconnect parts
Bleed brake systemBleed all wheel cylinders to purge air

If braking performance or pedal feel don’t seem improved, a problem may still exist somewhere in the hydraulic or vacuum brake system. Seeking professional diagnostic service at your local auto repair shop is the next step.

FAQs – Draining Your Brake Booster

Still have questions about properly draining your brake booster? These common FAQs provide helpful answers:

How often should you drain brake fluid?

Brake experts typically recommend flushing brake fluid every 2 years or 24,000 miles. Check your owner’s manual as some vehicles may specify more frequent brake fluid replacement.

What problems appear when you don’t drain brake fluid regularly?

Fading brakes, brake shuddering, leaking fluid, corrosion damage and other issues can crop up over time. Draining stale fluid prevents these problems.

Can I drive immediately after draining brake fluid?

Yes, but test brakes in a safe area first. Also be alert for some brake pedal softness initially as any remaining air bubbles work their way out through the hydraulic system.

What type of brake fluid should I use to refill?

Always refill your brake master cylinder reservoir using fresh DOT 3 or DOT 4 brake fluid. Avoid mixing different types of fluid.

Draining your brake booster takes a little time and effort, but keeps your brakes performing safely. Follow the steps outlined above to complete this important maintenance yourself.

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