Dabney Puts A Brake On A Project
Dabney was in a better than usual mood. He'd just finished waxing his 1938 LaSalle, and it was in its shining glory. So he decided to call Doctor Crankshaft.
"Hello," the Doctor shouted into the phone.
"Hey Doc, it's Dabney. I just finished waxing my '38 LaSalle and remember that you'd owned one just like it. Like to take a trip down memory lane? I'll bring it buy if you want to take it for a spin."
"Why Dabney, what a generous offer. I'd love to take you up on it," said the Doctor.
"I'll bring it around in about 5minutes," Dabney said and hung up the phone.
In five minutes the '38 LaSalle pulled up in front of Second Chance Garage.
"Dabney, my boy, this is terrific. I really appreciate it," said Doc.
"Well, Doc, you've helped me out with lots of stuff, so I thought I'd try to repay you some. Go ahead, climb on in." Dabney moved from behind the wheel to the passenger side.
The Doctor slid in behind the big steering wheel and adjusted the seat to a closer position, then put the old LaSalle in gear and let off the parking brake. Within seconds he had smoothly gone though all three gears and the car was cruising down the lane at a comfortable 35 mph.
"This old gal runs very nicely," said Dr. Crankshaft. "You know, these old Cadillac engines were strong and reliable as long as you took care of them."
"That's right, Doc, and this baby has over 100,000 miles on the odometer without any engine rebuild, and just listen to her purr," said Dabney proudly.
As the car rolled along, a sharp curve loomed ahead so the Doctor gently applied the brakes. "The brakes are a bit on the mushy side," the Doctor pointed out to Dabney.
"Yeah, I know, Doc. I noticed the same thing myself. These brakes were completely rebuilt five years ago and were as strong and tight as they could be. There's no way I've driven this car more than 1000 miles since then...maybe they need an adjustment," said Dabney.
"Five years," mused the Doctor. "You haven't put that many miles on them. Hmmm." The Doctor pondered the situation for a moment.
"What kind of fluid did you put in the brake system when it was rebuilt?"
"I put in some DOT 4 fluid from one of my SCCA race cars." Dabney said.
"DOT 4? Why DOT 4?" asked the Doctor.
"Well, I figured it was a better grade fluid than DOT 3, and I had some left over in the can and it made sense to use it up," replied Dabney. He saw the Doctor's frown.
"Well, boy, you've made two mistakes here. First, you used the wrong fluid in this old car and second, you used fluid from an already-open can," scolded the Doctor. "Let me explain something about brake fluid."
Dabney sat back in his seat and looked straight ahead at the road. He fully realized that Doc had a captive audience.
"There probably isn't any automotive component more neglected than brake fluid," said the Doctor. "As soon as the container is opened, glycol-based brake fluids (including all but a few specialized types) start absorbing moisture from the air. Once in the brake system the fluid absorbs moisture through microscopic pores in hoses, seals and air molecules imbedded in metal components like the cylinders and pistons. On average, moisture content in brake fluid reaches as much as 6-7 percent within four years."
"Here's where things get dicey," the Doctor continued. "As the moisture concentration increases in brake fluid the boiling point drops. As brake components heat up from friction in use, water vapor pockets can form, causing the brake pedal to go further down, causing a 'mushy' feel or downright failure. On top of that, the moisture corrodes the cylinder walls, lines and other metal components."
"You gotta understand about DOT ratings, Dabney," said the Doctor, firmly in charge. There really isn't any brake fluid used in cars that isn't at least DOT 3, so we'll start there. DOT 3 fluids must have a boiling point of 401 degrees F with no moisture content. You don't want brake fluid to boil, because it creates gases in the system, which are compressible and can cause the mushy feel and affect the brake system's performance. At a 3% moisture content these fluids must not boil at less than 284 degrees. DOT 4 fluids have a 'dry' boiling point of 446 degrees and a 'wet' boiling point of 311 degrees. DOT 4 fluids are more resistant to moisture absorption and their higher boiling points make them desirable in many front-wheel-drive/disc brake cars that create lots of friction when braking. But moisture in these fluids more drastically reduces the boiling points."
"It gets more complicated from here," continued the Doctor. "Brake fluid manufacturers add corrosion inhibitors and other chemicals to enhance the performance of the fluids, so just because a particular brand has a DOT 3 or 4 rating doesn't mean it's as good as another brand. For instance, Raybestos has a 'Super High Performance' DOT 3 fluid with a boiling point of 550 degrees F, much higher than several competitors' DOT 4 fluids."
"Then there's DOT 5 fluid, which is silicon based," said the Doctor. "It doesn't absorb moisture but brake system components can still corrode due to water in the air that was in them at manufacture. It does have a different viscosity than glycol-based fluids and it's much more expensive, so auto manufacturers don't typically use it."
"So why did the DOT 4 fluid cause the trouble in this La Salle," asked Dabney, his curiosity piqued.
"Because this car was built in 1938, when DOT 3 fluids weren't even invented and the brake drums were large enough to dissipate heat quickly," replied the Doctor. "DOT 4 fluid itself absorbs moisture more slowly, but a 3% moisture content reduces the boiling point by 50%! The fact that the can was open — thereby absorbing lots of moisture — meant that the fluid most likely had over 3% moisture content when it was introduced. You created the problem the moment you poured in the fluid."
"I guess I'll have to rebuild the brake system again." Dabney frowned.
"Not necessary, Dabney. I think a good flush of the DOT 4 fluid and a refill with fresh, unopened DOT 3 fluid will do the trick."
"I guess the next time I'll know to cross my T's and dot my DOT's, Dabney said, grinning.
The Doctor thought about coming back with something clever, but decided against it.