How To Bleed Brakes on a Hybrid Vehicle Part 1

How To Bleed Brakes on a Hybrid Vehicle Part 1

Some people automatically think that bleeding the brakes on hybrid vehicles is difficult, causes secondary service problems, or that doing so requires special OE tools and procedures including special scan tools.

While this may be true in some instances, it is not always true. In an effort to see just how simple or difficult it is to bleed some hybrid vehicles, we recently had the opportunity, on a limited basis, to use the Phoenix Systems Reverse Brake Bleeder on a few hybrid vehicles from a fleet we obtained access to. This fleet has a variety of hybrid vehicles and we chose a first generation Toyota Prius (Gen. I Prius) and a late model hybrid Ford Fusion to work on.

Why these vehicles? Because they are perhaps the two most likely that you may be servicing. The Gen. I Prius has been around long enough to be needing front brake work and the Ford Fusion by it's shear numbers will be into your shop for brake work sometime in the future.

We were able to use the Phoenix Systems Reverse Brake Bleeder attached at the bleeder valves at the wheels to bleed these two vehicles without doing anything special or using any special procedures. What we did was drive the vehicles into the shop, shut off the vehicles and close the door after removing the keys, lift the vehicle on the lift and attach the Phoenix Systems Reverse Brake Bleeder at each wheel and bleed (move new fluid from the caliper/wheel cylinder) at the wheel up stream to the master cylinder.

You will see that this was successful as the fluid level in the master cylinder was raised after each wheel was bled. Simple, effective, and successful. Will this work on all hybrids? Stay tuned as we continue to expand testing doing service work such as flexible hose replacement, caliper replacement etc. that will allow air into the closed system. What we did prove is that there is a straight fluid flow from the wheels upstream to the master cylinder and that we can move fluid (bleed) from the wheels upstream to the master cylinder on these two types of vehicles.

This week we will cover:
  • The history of the Prius used in this write-up
  • Where the master cylinder reservoir is and how to remove the cap
  • The importance of the master cylinder cap
  • A brief overview of the brake system
Hybrid Bleed 00 How to Bleed a Hybrid Step 00


Step 00

This Gen. I Prius has over 250,000 on it. All local in town driving and it's only on its second set of front brakes. The master cylinders fluid level when it came into the shop was very low. The reason for this was unknown. Also, the fluid appears to never have been tested for corrosion or changed. This alone would be reason do perform a fluid change and bleed out the system.

Hybrid bleed 01 How to Bleed a Hybrid Step 01

Step 01

The master cylinder on the Gen. I Prius is located approximately at the 11 o'clock position in this photo. It's about the only part of the hydraulic system that is easily visible. All other components of the hydraulic braking system are buried and difficult to see.

Hybrid bleed 02 How to Bleed a Hybrid Step 02

Step 02

This is not the master cylinder on a Prius. Stupid statement? You may be surprised to learn that some folks think it is and then add "you know what".

Hybrid Bleed 03 How to Bleed a Hybrid Step 03

Step 03

Removing the master cylinder cap is easy. Simply place your fingers under the rubber cap lip and pull out and upward. The reservoir is a white plastic container and has molded in fluid minimum and maximum markings on it.

Hybrid Bleed 04 How to Bleed a Hybrid Step 04

Step 04

The fluid level in the master cylinder was below the minimum mark and the bellows on the cap had expanded as designed. The bellows was pushed back into itself as shown.

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Step 05

Two things are important to understand about the master cylinder cap on this vehicle. One is that the specifications for the type of brake fluid are molded into the top of the cap. The other item is that you should realize that this cap is not a direct vent type of cap. The bellows which is part of the assembled cap separates the brake fluid from the atmosphere. This is the reason the bellows expanded into the master cylinder when the fluid level dropped. This feature prevents the brake fluid from constantly being in contact with air which contains moisture which would be absorbed by the brake fluid.

Hybrid Bleed 06 How to Bleed a Hybrid Step 06

Step 06

The calipers are conventional in their design and have a standard bleeder valve located at the highest part of the caliper.

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Step 07

This vehicle had the rubber caps on the bleeder so it was a safe bet they would open easy. Place a six point socket on the bleeder valve and open it with a ratchet.

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Step 08

The rear brakes on the Prius are drum brakes. The bleeders also opened easy as they also had the OE rubber caps on.

Hybrid Bleed 09 How to Bleed a Hybrid Step 09 

Step 09

A Phoenix Reverse Brake Bleeder and a couple cans of the correct brake fluid is all that was needed to bleed out this Gen. I Prius.

This will cover our portion this week. Come back next week for more information on bleeding a Gen I Prius with a Phoenix Systems Reverse Brake Bleeder.

Read part 2 of our blog post by clicking here.
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