A few weeks ago I accidentally left the parking brake on for a 25 mile drive, highway in the middle with about five miles on each end that were side roads.
On the last five to seven miles, the brakes went to the floor when I first pressed them, and that's the first time I knew there was a problem.
I figured out only then that the parking brake was on, which I released, but the brake pedal still went to the floor like there was nothing there.
I did have braking though, if I pumped the pedal on the early 2000s econobox FWD car, and I let it sit outside to cool off overnight.
In the morning the brakes were fine and they've been fine since then.
I'm just wondering what actually happened to make the brake pedal go to the floor, and then, when it cooled off, it fixed itself?
If the handbrake is integral with the service brake, you generated a awful lot of heat. The excess heat boiled the brake fluid and/or moisture content within the brake fluid. The boiled fluid gives off
*steam* and this steam, being in a gaseous state, is compressible - hence your pedal travel to the floor. As you have noted, it can be pumped up so you have a pedal but it will be very spongy. When the brakes cooled down, the steam condensed back to a liquid and and your pedal returned to its normal height.
While you say everything is back to normal now, I would definitely be thinking about changing the brake fluid. Brake fluid normally has a very high boiling point, typically 250C, so will not boil under most circumstances. Unfortunately, brake fluid is also *hygroscopic* which means it has an affinity for water and will absorb moisture readily - even from the air if left exposed. You may well have moisture contaminated brake fluid which has its boiling point lowered down as far as the boiling point of water - 100C. That's bad. The moisture content will also cause corrosion of internal brake components. This is why you need to change brake fluid every couple of years.
Thank you for explaining that the brake fluid turned to steam which is compressible. I'm surprised the rubber lines did not fail given the heat. I did check the ground by parking the car outside on clean concrete and there were no drips. I also checked the master cylinder fluid level which seemed OK.
I'm surprised with all that steam that the brakes worked at all when I pumped them, but they worked well enough, where I had to plan ahead to brake by pumping but it only took one or two pumps to get some braking back.
But that first pump was totally resistance free! That's scary.
I think it's a good idea to change the brake fluid which I can do. I don't think this even sucked up water because it's a closed system, isn't it?
Sometimes heating things can ruin them. Are you saying that heating brake fluid ruins it?
I'm only asking to learn as I agree fully that I should change the brake fluid anyway as I got the car with an unknown history.
Everything, when cooled, returns to the status quo.
It is what brakes are designed to do. You are, in effect, using the MC as a pump and compressing the air so it acts on the fluid and, in turn, acts on the SC pistons.
That's when you really understand the term; *arse pucker*.
It is a closed system but moisture can still be absorbed through hoses, past piston seals, etc. Every time you take the MC cap off to check the fluid, you expose the fluid to moisture laden air. That is why most MCs have see through reservoirs - no need to remove the cap to check fluid levels. That said, I have seen reservoirs develop a *stain* that can be a trap for young players as they think that is the level whereas, in fact, the actual fluid level can be low or non-existent.
The heating, and subsequent vapour lock, tends to indicate the brake fluid may be moisture laden rather than being damaged by excesses of heat. If the vehicle is an unknown, then change the fluid as a matter of course. That way you can then *know* the condition. What you should do is avail yourself of a brake fluid test kit and see for your self what the moisture content of the current brake fluid is.
Note, when bleeding fluid through the brakes, do not depress the brake pedal to the floor. Depress it only as much as the normal travel would have been. This is because the bottom end of the MC bore may have a corrosion or similar build up which can damage MC seals. That said, your MC piston has been to the bottom of the bore in your brake vapour lock incident anyway.
Thank you for your advice and explanation of why when I replace the brake fluid I don't want my helper to pump the pedal all the way to the floor.
I'll use a turkey baster to suck the yellow fluid out of the master cylinder (leaving just enough old fluid to cover the opening), fill the master cylinder near the top with the blue fluid (leaving enough empty not to spill), and then crack open the brake line with those wrap around open end wrenches at the rear furthest wheel first, and have the helper go "almost" to the floor each time until I see blue fluid consistently, and then move on to the next furthest from the driver in turn until I get to the driver side front wheel.
Yeah, advice a bit late for your case since your pedal has already been to the floor but worth keeping in mind for future bleeding tasks.
I have the proper suction pump *dedicated* for that purpose - reduces cross contamination risk. You might want to consider one of the bleeding kits with a one way valve - makes brake bleeding a one man task.
Yes except you should be cracking open the bleeder nipples. The bleeders are, or should be, at the high point in the calipers/wheel cylinders and, as a consequence, will be better placed to remove air, if any, from the system. Cracking open pipe connections will not guarantee old fluid is removed from the calipers/wheel cylinders. Disturb *pipe connections* only when necessary and, for bleeding, it is rarely necessary since they are often *not* at the high point. Be sure you don't slip up and run short of fluid in the reservoir. Air in the ABS system can be a bugger to remove.
I like to learn so it's good to know. Thanks for letting me know the pedal should never go down to the floor on purpose due to the corrosion on the end of the rod damaging the master cylinder parts.
You have a point on the cross contamination which I had not thought of. I will get a plastic turkey baster at the dollar store for a buck and then I can throw it out afterward (or label it for brake fluid only).
Thanks for the cross contamination heads up.
That's what I meant. :)
I put a 10 inch or so soft hose on the bleeder valve and then into a glass jar so I can see the color of the fluid change from yellow to blue.
Thanks for the heads up. I don't think this early 2000's econobox car even has ABS, but it might have it. I never felt the pulsation though so I don't think so. But I'll be careful not to run the master cylinder dry.
I guess that's where the third person can come in handy! :)
Thanks for posting the problem, since you had to confess you are not perfect. I sure ain't. you remind me of my 64 beetle in the German mountains having brake fade. I remember that feeling. One of my current brake problems is assuming every shop is crooks. a few years ago I had some simple service done that cost ~10x more than it should, and on top of it they committed life threatening sabotage, over tightening the spindle nut.
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