On 26/2/20 11:37 am, delvon daily wrote:
> Xeno wrote:
>
>> 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.
>
> Sorry for the doublepost mistake.
>
> 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.

Everything, when cooled, returns to the status quo.
>
> 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.

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.
>
> But that first pump was totally resistance free!
> That's scary.

That's when you really understand the term; *arse pucker*.
>
>> 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.
>
> 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?

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.
>
> 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.
>
> Does heating in and of itself damage brake fluid?
>
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.


--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)