Posted on: February 5, 2016

by Tom Ehrhart in the March 2002 RootesReview:

When should you use Silicon vs. Girling Brake Fluid?

Use silicone brake fluid for street driving and occasional collector car type use and use Girling brake fluid for racing applications.

Why?

Both fluids have their virtues and liabilities. For the owner, it is a matter of compromises that require choosing the lesser of the two evils for their application. With respect to the typical street driving applications, both fluids will eventually cause seals to degrade. The choice in my opinion is, which fluid causes the less severe failure. With silicone it is only the seal. Repair is simply, disassemble, clean, and reassemble. With Girling, it is the seal PLUS, likely cylinder bore damage from corrosion, ruined paint from leaks, and rusty floor boards from fluid leaking out of the master cylinders. It is ugly. Repair requires seal replacement, reboring or sleeving, and heaven knows what to do with the paint and rust damage.

Racers are well aware of the nasty chemistry of Girling brake fluid that dictates regular changes and the care associated with handling the fluid.

From a performance perspective, in my opinion the compressive characteristics of silicone (soft spongy pedal, especially when hot) are not an issue under even aggressive street driving. I have never had silicone fluid problems when towing with the Tiger; which produces far greater brake stress than any Tiger driven on the street. Girling brake fluid is the only way to go with the relentless braking that occurs during high-speed road racing.

Bleeding Precautions

One caution I would issue regarding silicone is during the bleeding process. It is important that you never pull a vacuum or pressure-force fluid through the lines. The vacuum will cause the fluid to vaporize and cause air bubbles to form in the fluid. Applying too much pressure will cause the fluid to flow at a high velocity, causing it to cavitate or agitate the fluid to a point where bubbles develop. In either case you will have residual air bubbles suspended in the fluid in the lines and cylinders.

When using the master cylinder to force-bleed hydraulics, be sure to depress and release the pedal SLOWLY. Pushing too fast causes high pressure. Releasing too fast produces a vacuum in the master cylinder, causing bubbles to form so that the next time the pedal is depressed fluid with air bubbles is now pushed into the hydraulic lines. Have you ever wondered why the system would never bleed? Gravity bleeding works the first time every time in an Alpine or Tiger. I do force bleed on occasion by securing the cap on the reservoir and applying about 1 or 2 pounds of air pressure, but no more. It’s a different story with some cars like the 69 Alpine. This system requires selective front and back bleeding because of the servo, balance sensor and dual master cylinder arrangement.

Editors note: As of 2002, the “Girling” brake fluid discussed in this article is sold under the name of Castrol GT. It also carries the names of Lucas and Girling and is rated as DOT 4 LMA Brake Fluid. The silicon brake fluid is sold under several names and is rated as DOT 5 Brake Fluid.

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