During a recent service of my 2004 XK8, the mechanic did not replace the brake fluid by bleeding it all the way through to the wheels. Instead, he just sucked all the fluid out of the reservoir using a special pump and then refilled. In his opinion, this was good enough because he thinks that any moisture entering the system settles out in the reservoir.
Do any members know whether this is correct?
Graham Barker

 

No. The moisture spreads throughout the system.

 

+1

Brake Fluid (other than silicone based fluids) is hygroscopic meaning that it absorbs moisture from the air. This disperses throughout the fluid and doesn't settle out like cream on the top of the milk. The consequent risk comes from the fact that this reduces the boiling point of the brake fluid.

There's two associated issues result from this moisture content:

1. under hard braking, the fluid can boil leading to seriously reduced braking efficiency or even loss of brakes
2. the moisture causes internal corrosion of brake system components

The fluid that's going to get hottest is the fluid in the calipers. Draw your own conclusion.

To expand on the flushing / bleeding procedure. The reason for bleeding fresh fluid through from the reservoir to the calipers is to remove contaminated fluid. As the brakes are used, there is inevitable wear on the rubber caliper seals and minute particles of rubber become suspended in the fluid. This is why the first fluid bled from the calipers often looks dirty.

Graham

 

The moisture in the system magically migrates to the reservoir?

Um, no, I don't think so.

Contaminates in the system wouldn't migrate anywhere either.

It is recommended that all the brake fluid be replaced every two years. All of it, not just the few ounces in the reservoir. When I did mine it took a quart to thoroughly replace all the fluid.

I would like to hear what your mechanic has to say about this now.

Or maybe you should find another mechanic who doesn't believe in unicorns.
Vector

 

. . . and the fluid doesn't circulate except for the minute amount transferring pressure. What is in the caliper chamber stays in the chamber and that is where corrosion damage does the most harm. You must have gotten the $5 special.

 

Modern automotive techs are, in general, better educated and informed than the greater population. Still, one has to be alert to areas of breath taking ignorance and intellectual blind spots, no matter who you are dealing with.

I'm so thankful that ignorance and intellectual blind spotting is not part of my make up.

 

+1. Typically when blakes are bleed the most visibly contaminated fluid emerges first from the calipers.

 

Thanks, guys. I will be having a serious talk to the mechanic.
Graham Barker

 

Please don't drive that car. Have it towed to another mechanic who will bleed it properly.

 

This is EXACTLY why I prefer to do all of my own work if possible.

 

At least that mechanic told you what he did!

-Dave