Adler's Antique Autos, Inc.
Author of "Notes from the Corrosion Lab"
801 NY Route 43, Stephentown, NY 12168
(518) 733 - 5749 Email
by Bob Adler
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.
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
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
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
Q: “I run a vacuum booster and silicone fluid,” writes reader Jack Grosskopf. “Should I change 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.
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.