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If you’re in charge of ordering the supplies for your shop, volume discounts are great when they come along. Sniff them out where you can. But brake fluid is different! Buy it in bulk, but buy it in the smallest containers you can get away with. The reason? Brake fluid is hygroscopic. And the boiling point matters more than ever.
To wit: if you read most any master cylinder cap, you’ll be instructed to use brake fluid from a sealed container. That’s to ensure that the fluid doesn’t suck moisture out of the ambient air. (That’s what “hygroscopic” means.) When brake fluid becomes waterlogged, the boiling point of the fluid is lowered, and the water can actually boil off. Fluid, as we learned from Pascal, is incompressible, but that’s not true of a gas (like steam). If the water turns gaseous, the pedal can travel without compressing the fluid and then moving the pads or shoes, which is obviously not good.
Couple this with the rise of vehicles using DOT 4 brake juice. This differs from the more common DOT 3 only in terms of having a higher boiling point. Why? Brakes are being asked to do more work that generates more heat, and the hydraulic fluid needs to work harder as well. That fact, coupled with ever-more-intricate ABS systems with faster processors and controllers, means it’s more important than ever to keep water out of the system.
Buying small, sealed containers can help you do this. Sure, if you’re finally plumbing up that old project car, buy a quart or even a gallon for the bench bleed on the master and gravity bleed of the lines and hoses that will follow. But you’ll want to toss it not long after, and for most regular fluid swaps done in tandem with a brake job, a pint or two will do you just fine.
The scant savings from buying in bulk might not be all that prudent. So get your oil in a drum—but brake fluid should be in airline bottles only.
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