Over the past few weeks we’ve looked at an that loosens bolts in tight areas, a to restore damaged threads and a cheap that actually works. This week’s cool tool is a brake fluid tester pen that can help you maintain your car’s braking system.

This suggestion comes from our Deputy Editor, Norman, and I thought the idea was so cool that I bought one to test out:

It works by testing the moisture content of your brake fluid. Why should you care about how much moisture is in your brake fluid?

Brake fluid is hygroscopic; it absorbs moisture from the environment around your car. Moisture can enter from the master cylinder reservoir lid or anywhere that your brake system isn’t perfectly sealed. Too much moisture and you’re looking at corroded parts and reduced braking performance. Contaminated brake fluid can also boil under heavy braking. These are all sorts of not-good situations, so having water-free brake fluid is essential.

Many auto manufacturers recommend replacing brake fluid every two years to ensure good braking performance and longevity of braking parts. This recommendation is good, but it isn’t perfect. As magazine, which writes about car tech for repair shops, notes, a vehicle working in a hot, humid environment could see its brake fluid age much quicker than other vehicles. So it doesn’t hurt to test your fluid every now and then.

This brake fluid tester pen works by measuring the electrical resistance of brake fluid.

Moisture changes the resistance of the fluid, resulting in a different reading on the tester. These pens are great because unlike testing strips, they can be used an unlimited number of times.

To test how this cheap unit from Amazon works, I decided to test the brake fluid in a .

This car works hard, adding hundreds of miles to its odometer almost daily. When it was purchased back in November, the previous owner stated that he recently changed the brake fluid. Let’s see how that fluid aged in the 12,000 miles since then.

Setting up the tool is easy: It needs a single AAA battery and it operates with the push of a button. Dip the probes into the fluid and the tool should quickly produce a result.

A graph printed on the tool tells you what it means when a certain LED is illuminated.

If moisture content is at three percent, it should be changed. If moisture content is above 4 percent, it should be changed immediately. This is in-line that shows that the boiling temperature of DOT 3 brake fluid drops by 100 degrees with only 3 percent of moisture content. Get as high as 7 percent and the boiling temperature can get near the boiling point of water.

I popped the cap off of the tester pen then inserted it into the HHR’s brake fluid reservoir. It came back at a safe 1 percent or less.

Just to make sure that this actually works and doesn’t give entirely wacky readings, I also tested a cup of water.

Wow, water has a huge moisture content!

There is one potential downside to this tool. As Brake & Front End notes, some contaminants in the brake fluid can also change the electrical resistance of the fluid and thus make the tester give a false reading.

These tools run really cheap. The ITEQ model that I purchased from Amazon was $13 and you can get one for under $20 on just about any site.

Do you know of a weird or unique but must-have tool that every wrencher should have? Do you want to see us put a type of tool to the test and see how it performs? Shoot me an email or drop it down in the comments!