Thanks for the replies fellas, much appreciated.

John

When I attend to the brakes I have the whole system 'overhauled', not just the fluid changed. Every 6 years or so. I don't think brake fluid deteriorates much in normal use, tho it is said to be hygroscopic. That should not pose a problem unless you somehow drive so maniacally that your brakes get hot enough to boil the absorbed moisture in the fluid.

In my case, what seems to have happened is:-

a wheel cylinder leaked, due to internal corrosion.

This caused a brake shoe to detach.

The brake shoe wedged under the other brake shoe and heated the brake drum to "maniacal" levels without any maniac action on my part.

This boiled the ancient, saturated brake fluid, resulting in total, sudden, pedal to the floor brake failure..

No maniacs were harmed during the production of this incident.

Firstly, complete drainage of the old deposit laden oil adhering to the interior metal like the sauce on the sides of a sauce bottle, by leaving it for several hours to drip into a tray before refilling the sump.

There's one engine I read about, where you shouldn't do that. It relies on the residual oil to prime the pump.

and renewing the brake fluid when you have your car serviced ?

I'm in the 'hardly ever' camp. I suspect our 21yr old 21st century Focus went to its grave with much of its original brake fluid present. Changing it every few years is 20th century advice for 21st century cars. Modern DOT 4 brake fluid has a significantly higher boiling point than its predecessors and the sealing of modern brake systems and their reservoirs ensures that there is effectively no exposure to moisture.

He said that the reason that they don't drain off all the brake fluid is because there is a danger that the bleed valve or nipple could be corroded and easily damaged, this could cause a much bigger problem so they leave well alone.

Nonsense - although cack-handed mechanics could mis-thread them or overtighten them and strip the thread. The real reason is probably because it takes time, which costs. Indeed, there is a very good argument to loosen the bleed nipples every five years or so and press out a few mls of fluid into a jam jar, not only to inspect it but mainly also to ensure that the nipple and caliper threads are not so welded together by age and corrosion that the head breaks off when trying to undo it. In the old days of poor seals you would often expel a grotty contaminated watery dark brown substance. This century I have always noted clear uncontaminated fluid on the rare occasion I have inspected it.

A few counter-arguments, some of them tentative, some bleedin obvious.

I dunno if the "20th century advice for 21st century cars" is truly a bad thing, since I wouldn't much thank you for a 21st century car, but judging by the sorry state a 1999 Honda Accord I brake flushed was in, I'd be surprised if a 2000 would be much better.

Particularly difficult reservoir to flush, with lots of sludge traps. Dunno why. Maybe to stop the sludge getting deeper into the system.

Even if it were truly a bad thing, there are still a few 20th century cars around. I believe you have one, for example.

Re DOT 4 brake fluid having a higher boiling point, I believe this is true, but it is also more hygroscopic than DOT3, (The CPC DOT4 comes in a metal bottle, the CPC DOT3 in plastic, probably for this reason) as well as being twice the price. Wet brake fluid causes corrosion, so the boiling point isn't the only criterion.

It is clearly NOT "nonsense" that brake nipples can corrode, seize, and shear off, and to be fair, having said its nonsense, you then go on to directly contradict yourself. They are small, relatively weak (being hollow), and in an exposed position, so if they are not explicitly protected from corrosion (and rather few punters or mechanics are going to do that) failure is hardly surprising.

Edited by edlithgow on 18/02/2024 at 04:19