Reserve Alkalinity

First of all, what is "Reserve Alkalinity?"

Simply put, "Reserve Alkalinity" is a measure of the quality and "robustness" of the brake fluid’s additive package as well as being an indirect indicator of the overall quality of the materials blended to create the brake fluid. This additive package, which all glycol, borate ester, and silicate ester based brake fluids (this covers all DOT 3, DOT 4 and DOT 5.1 brake fluids) have, provides the brake fluid with the following:

  • corrosion inhibition
  • buffer against the material’s (the various components that make the brake fluid) propensity to go acidic over time and use,
  • lubrication
  • low temperature viscosity

OK, now with that said let's start at the beginning so this can all make sense. So to start we need to understand some fundamental realities of brake fluid. And let's also agree at this point that when we say "brake fluid," we are referring to all DOT 3, DOT 4 and DOT 5.1 brake fluids. We are NOT referring to the mineral oil or silicone based (DOT 5) brake fluids.

Be aware that the Reserve Alkalinity measurement is NOT something you will find in the SAE J1703, J1704, J1705 brake fluid standards or in the Federal Motor Vehicle Safety Standard (FMVSS) #116 which covers the DOT requirements for all DOT 3, DOT 4, DOT 5 and DOT 5.1 class brake fluids.

All brake fluids whether glycol, borate ester, or silicate ester based and their derivative materials are corrosive. Their corrosive nature is due to the degradation of the materials into organic acids in the presence of oxygen and heat. Because of this fact, all brake fluids must include an additive package which contains an inhibitor formulation to prevent corrosion to the system by the acids. The inhibitor formulation typically contains a buffering agent and metal passivators that functions to buffer the acids as they are formed, and passivize metal surfaces to control corrosion.

The specific term used to describe this buffering is "reserve alkalinity." Reserve Alkalinity is measured as the number of milliliters of N/10 hydrochloric acid needed to titrate 10 milliliters of glycol to a pH of 5.5. In its most simple form, the "Reserve Alkalinity" number of index is an indicator of how resistant a fluid is to becoming acidic.

There are several other items of which the Reserve Alkalinity "index" can be an indirect indicator. For example, there is a viscosity standard for brake fluid measured at -40 C (-40 F) and also at 100 C (212 F) For the most part the 100 C data point is nothing to worry about. Brakes fluids are able to comply with that temperature/viscosity requirement rather easily. However with high performance/racing brake fluids where there are concerns and performance objectives as to reducing compressibility and increasing the "pedal feel" of the fluid.

Let us quickly review the DOT 4 standards in regards to viscosity as it is the most common standard used for comparison purposes. The DOT 4 standard calls for a maximum viscosity of 1800 cSt (centistokes) measured at -40 C (-40 F). With this in mind a great example of where product development goes bad is witnessed in the Brembo HTC-64T brake fluid. HTC-64T is Brembo's newest brake fluid and has been marketed to race teams as a "racing" only product. In our independent laboratory testing, we noted a -40 C viscosity of 2814 cSt! This is 57% in excess of the DOT 4 standard.

Why would the brake fluid vendor to Brembo engineer this exclusive brake fluid in this manner? With the quest for higher and higher dry boiling points the chemical engineering becomes more complicated and expensive. Therefore, there is no motivation to really do all the engineering to do things "right." First off, as the relationship between the two viscosity data points is not linear, allowing a very high -40 C viscosity allows for a better pedal feel. Second, the additive package is where this "off the scale" viscosity would be brought into spec. However the "jury rigged" engineering as demonstrated in this brake fluid also demonstrates itself in the way of the Reserve Alkalinity results.

So how does Reserve Alkalinity apply here? The Reserves Alkalinity for this brake fluid is only 25, a rather low measurement. So in this case, the low Reserve Alkalinity measure is an indirect indicator of what ultimately would be considered a low quality brake fluid. Now, it is also the case that the Brembo HTC-64T also tested at a pH of 6.5 i.e. below the standards minimum of pH 7.0, making the HTC-64T an already acidic fluid. The last thing you want are your nice metal brake calipers and master cylinder(s) filled with an acidic fluid eating away at the metal!!

To put this into perspective, here are the Reserve Alkalinity results for several other brake fluids.

Brake Fluid
Reserve Alkalinity
  Brake Fluid
Reserve Alkalinity
Endless RF-650
  Brembo HTC-64T
Castrol SRF
  Motul RBF600
Torque RT700
  Motul RBF660