Are resistance/capacitance based brake fluid testers reliable?

Question

I would like to test my brake fluid to see whether it needs replacing rather than spend the time and money on it unnecessarily. I've seen inexpensive brake fluid testers on the market which work on the basis of detecting water percentage based on electrical resistance or capacitance. These devices have 2 prongs which you stick into the fluid reservoir and it lights up different LEDs depending on how much water it thinks is in it.

Are these types of testers reliable on to give an accurate reading? I've seen other types of tester but many of them cost more than replacing the fluid, are there other inexpensive but better options?

Answer 1

The reason that brake fluid should be replaced on a regular basis (every two years is a common recommendation) is that brake fluids are hydrophilic (they "like" water) and over time atmospheric humidity builds up in the fluid. When this happens the fluid can boil under heavy braking, which would cause the loss of braking power. Moisture in the fluid can also result in internal damage to the brake system.

Those are big enough risks that you'd want to be really sure of a few things before trying to extend the life of brake fluid based on the readings:

1. What actually happens to the moisture picked up by the fluid – if it is evenly dispersed in the fluid then the meter might work, but if it accumulates in low areas or if it is not necessarily evenly dispersed then you may well get a false reading from the meter. A false positive could be very bad news.

2. How do you interpret the meter readings? For example, how much moisture is tolerable? Is it the same for all fluids? How reliable are the readings?

All in all, replacing brake fluid seems like cheap insurance to me and without some good and reliable information about the effectiveness of the meters I'd be inclined not to trust them.