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Silicone Brake Fluid
Part 2
by Bob Adler

Notes From the Corrrosion Lab.In Part 1, we discussed some pros and cons of switching to silicone brake fluid, especially compatibility of silicone with brake-component elastomers.

One concern with silicone fluid is that it may not sufficiently lubricate the sliding pistons in a brake system. The Society of Automotive Engineers (SAE) has a cylinder stroking test designed to evaluate the lubrication quality of glycol DOT 3 brake fluid. This is a good, practical test for predicting if a brake system will work for a few years in the field, but it is based on glycol test fluid.

The SAE's “Surface Vehicle Recommended Practice J1705” can be used for silicone DOT 5 fluid. This recommended practice runs the same tests as the SAE J1703 standard discussed in the Part 1, but substitutes silicone fluid for glycol. It is a recommendation, not a standard, so it does not carry much weight in the industry, but it does offer interesting guidelines for brake-cylinder evaluation and rebuilding.

The SAE tests recommend that you “disassemble cylinders and discard rubber cups. Clean all metal parts with isopropanol or ethanol and dry with clean, compressed air. Inspect the working surfaces of all metal parts for scoring, galling, or pitting and cylinder-bore roughness and discard all dcfective parts.” The instructions recommend that we inspect new cylinders before installing them on our trucks to make sure that they are in good shape. Remove any stains on cylinder walls with crocus cloth and isopropanol or ethanol. If stains cannot be removed, discard the cylinder,” says the SAE.

When rebuilding a cylinder the questin is, “How far can I hone a cylinder?” always comes up. “Select parts to insure that the clearance between each piston and mating cylinder is within 0.003-to 0.005-inch,” says the SAE. So, if honing a used cylinder results in more than a 0.005-inch clearance (measured with a feeler gauge) to the piston, the cylinder should be sleeved or replaced. “Use new SBR cups that are free of lint and dirt. Discard any cups showing imperfections such as cuts, tooling marks, molding flaws, or blisters. Clean rubber parts with isopropanol or ethanol and a lint-free cloth. Dry with clean, compressed air. Dip the rubber and metal parts of the wheel cylinders, except housings, in the fluid to be tested and install them in accordance with manufacturer's instructions,” says the SAE. Denatured alcohol, which is readily available, is mostly ethanol. Rubbing alcohol is 70 percent isopropanol and 30 percent water.

Bad News
The Styrofoam-cup test for glycol brake fluid reported in part 1 did not work for me. The cup never disintegrated with glycol fluid. To determine if an unknown sample is glycol or silicone fluid, put some in a glass jar. Add water a drop at a time. Glycol fluid will dissolve in water with slight milkiness, but silicone fluid will form two distinct layers, with water on the bottom. Silicone stays clean and clear in a brake system, but old glycol looks rusty after years of use. The purple dye transfers from silicone to glycol fluid if you mix them together.

Military departments have been using silicone brake fluid for many years. Let's see if I can get any information from them for the next issue.

Summary So Far
Choosing a brake fluid, machine shop, chrome-plating shop, physician or spouse is a really personal decision.

One of life's personal pleasures is looking inside a master-cylinder reservoir with clean, rust-free surfaces after ten years of silicone fluid use. Our main concern with brakes is that after a complete rebuild, they should work reliably for a decade or two before needing another rebuild. One way to extend this rebuild interval is to change glycol fluid every few years. If your maintenance schedule is likely to become lax, silicone fluid should not need changing as often.

If you use new name-brand components and fluids that meet specifications, silicone fluid is a good bet.

Beware of silicone leaking through a vacuum booster and scouring the engine. Performance DOT 4 fluid may have a higher wet boiling point because it absorbs less moisture. This is good, as less moisture means less corrosion and longer maintenance-cycle intervals.

Racing fluids may not have anticorrosion additives

Stroking Test
This test is part of SAE Standard J1703, Motor Vehicle Brake Fluid (for glycol). In this test a master cylinder is connected to three wheel cylinders and an actuating mechanism capable of providing 1,000 strokes per hour at 1,000 psi system pressure. The test is run for 16,000 cycles at 23 degrees Celsius, then the temperature is raised to 120 degrees Celsius as stroking continues, for a total of 85,000 strokes.

Cups, fluid, and metal parts are then inspected. Standard SBR cups are inspected for tackiness, scoring, scuffing, blistering, cracking, chipping, and change in shape from the original appearance. Cup hardness and diameter changes are noted. Brake fluid is collected in a glass jar. “The fluid at the end of the test shall not be in an unsatisfactory operating condition as evidenced by sludging, jelling, or abrasive grittiness, and sedimentation shall not exceed 1.5 percent by volume. . . . Metal parts shall not show corrosion as evidenced by pitting to an extent discernible to the naked eye, but staining or discoloration shall be permitted. The initial diameter of any cylinder or piston shall not change by more than 0.13 mm (0.005 in) during test. The cylinder pistons shall not freeze nor function improperly throughout the test.” The test also specifies how much fluid can be lost, and notes that a trace of gum is permitted and that cylinders “shall be free of deposits which are abrasive.” (back to article)

Wet Boiling Point
Both SAE J17O3 and J1705 and FMVSS 116 give specific lab procedures to determine the wet boiling point. A standard reference fluid (triethylene glycol monomethyl ether) and the test brake-fluid sample are each placed in a humidification chamber. When the standard fluid reaches 3.7 percent moisture, as measured by another standard test, the wet boiling point of the test brake fluid is measured.

Minimum Boiling Point Chart
Product Dry Boiling Point* Wet Boiling Point**
Valvoline SynPower®
502 degs. F
343 degs. F
401 degs. F
284 degs. F
446 degs. F
311 degs. F
500 degs. F
356 degs. F
DOT 5.1
518 degs. F
375 degs. F

* Dry boiling point - Temperature at which brake fluid boils with no water present in system.
** Wet boiling point - Temperature at which brake fluid boils with 3 % water by volume in system.

Corrosion Test
FMVSS 116 and SAE J17O3 and J1705 all have the same corrosion test. In this test clean, shiny test strips of various metals used in brake systems are put in a jar with a mix of brake fluid, water, and wheel-cylinder cups. J1705 says to use humidified fluid. Bake at 100 degrees Celsius for 120 hours. “Examine the strips for evidence of corrosion and pitting.” The weight of each strip must not change by more than a specified amount, indicating the amount of corrosion. “Examine the cups for evidence of sloughing, blisters, and other forms of disintegration. Measure the base diameter and hardness of each cup” before and after the test. “Examine the fluid-water mixture in the jars for jelling.”

Reader Questions

Q: “I run a vacuum booster and silicone fluid,” writes reader Jack Grosskopf. “Should I change to glycol fluid?”
A: If all brake components are relatively new and not leaking, I recommend leaving everything in place. Check for seepage in the vacuum hose to the intake manifold by swabbing with a clean white paper towel. The purple dye in silicone fluid may not be strong enough to show up on the towel, so just look for dampness that is not gasoline. Your nose will tell you the difference. Make this a part of your periodic maintenance. If the booster is mounted directly to the master cylinder, loosen the master cylinder from the booster and check for leakage between the two components. If the master-cylinder secondary cup leaks, it allows brake fluid to enter the booster. At the first hint of silicone leakage in this area, it is time for a new booster and/or new master cylinder, or conversion to glycol fluid.

Anyone planning to buy a vacuum booster should ask the technical departmcnt of the manufacturer if its elastomers are silicone compatible. Answers from the sales department are usually vague and unreliable, but if you can get through to product engineering, the answers get much more precise. Ask if there is any guaranty difference with silicone fluid. Ask if the clastomers meet SAE J1705 specifications for silicone fluid. Personnel in the sales department might not know the difference between J 1703 and J 1705, but they should in engineering. You should note that directions to convert to silicone fluid always recommend using all-new elastomers. I know of no published directions to convert from silicone back to glycol fluid. Logic would dictate using all new elastomers at this point, but if the silicone installation has been successful for many years, why not just replace the leaking component and continue using silicone fluid?

Q: “What is synthetic brake fluid, and how does it affect paint?” another reader asks.
A: Valvoline makes SynPower® synthetic-formula high-performance DOT 3 and DOT 4 brake fluid. DOT 4 brake fluid is glycol based, similar to and compatible with DOT 3, but it has a higher wet and dry boiling point. The bottle label says, ‘SynPower® high performance synthetic brake fluid is recommended for ABS, hydraulic drum and disc braking systems, and hydraulic clutches requiring DOT 3 and DOT 4 fluids. Its premium, synthetic, low-moisture formulation provides superior anti-vapor lock protection. Has a high dry boiling point up to 502 degrees Fahrenheit that reduces the risk of fluid vaporization and possible brake failure. Meets and exceeds specifications SAE J17O3, Federal Motor Vehicle Safety Standard (FMVSS) 116, and DOT 4 Motor Vehicle Brake Fluid requirements. Mixes with conventional brake fluids. Wet boiling point of 343 degrees Fahrenheit. . -. Contains: Brake Fluid (a complex mixture of glycol ethers, polyglycols, and inhibitors).”

I called Valvoline and asked which part of the formula was synthetic. The reply was that these glycols are not naturally occurring molecules, so they are synthetic. Emphasizing “synthetic” seems to be a marketing technique rather than unique chemistry. The advantage of using SynPower® is its high boiling point, both wet and dry (see sidebar), which bumps it up to DOT 4 specifications. Valvoline claims low-moisture formulation. While boiling brake fluid is not a primary concern of vintage trucks and truckers, a fluid that absorbs moisture slower than average could translate into less corrosion in the brake system over a longer time period.

Paint will still blister upon contact with SynPower® brake fluid. Have a water bucket and sponge ready before starting brake work to immediately dilute and wipe up spills.

I also asked about corrosion inhibitors in brake fluid. Valvoline said alkalis and buffers are used to prevent corrosion. This is important at the outer edge of brake cylinders, where atmospheric moisture interfaces with the brake-fluid film. Even with its anti-corrosion package and low-moisture-absorbing formula, Valvoline still recommended changing brake fluid every two years, which it felt was about the frequency of brake repairs on vehicles driven daily. The company also said moisture diffuses through the flexible brake hoses at the axles. The anti-corrosion additives in brake fluid can be used up, much like the additives in engine oil, which are changed at that point. It sounds like a good argument to change glycol brake fluid whenever performing other brake or axle work.

Bob Adler is owner of Adler's Antique
Autos, Stephentown, New York, and
specializes in GM truck restoration.
He can be reached at 518-733-5749.
Adler's Antique Autos