Converting from DOT 3 brake fluid to DOT 5 fluid

Question:

l want to change the glycol-based DOT 3 brake fluid in my 1957 DeSoto to silicone DOT 5 fluid. I was told by an ASE certified mechanic this should not be done. I was then told by the owner of a brake specialty shop this could be done, but the brake lines need to be replaced and the master and wheel cylinders need to be disassembled and thoroughly cleaned. And finally, I was told by a local auto supply’s question-and-answer service I merely had to open all the bleeders, allow gravity to drain all the DOT 3 fluid and then add DOT 5 fluid to the master cylinder. When the DOT 5 exits all the bleeders should then bleed the entire system to complete the job. Which of these opinions, if any, is correct?

Answer:

I have installed silicone brake fluid into scores of older cars (including a 1957 DeSoto!) and, based on my experience and’ knowledge, none of the advice you received is totally correct. Glycol-based fluid and silicone fluid are not compatible with one another. When the two are mixed together it is likely that air bubbles will form. Therefore, it is imperative that all of the old fluid be removed before the new fluid is introduced. Flushing out the old fluid with the new will not ensure that every drop of the old fluid is removed. The proper way is to remove all four wheel cylinders and the master cylinder. Disassemble and thoroughly clean each component. Rebuild any of parts that need it. You do not need to replace the brake lines if they are in good condition. Simply flush them clean with denatured alcohol, Permatex Brake Clean, or a similar solvent, and then blow them out with compressed air to remove the solvent and leave them clean and dry. Then reinstall the Wheel cylinders and master, add silicone fluid and bleed the entire system.

You will likely need to bleed the system more than you would with conventional fluid because silicone has a greater tendency to hold air bubbles in suspension.