Thread: Replacing rear pads and discs. Any traps for the unwary?

Originally Posted by johntins

Wow. Various reasons, huh? Pad wear; normal. Anything else; abnormal. Leaks, fractured hoses, hmm, what else? Oh, temperature; sure, that will change it if you have a microscope. If you want to play around with your safety that's your lookout. As someone who qualified as a motor mechanic, albeit a long time ago, I think I'll take my advice over yours. This particular comment is welcome to be read by geese.

A goose wrote it.

Originally Posted by rick130

Umm, my Defender used to do exactly the same and cause the low fluid light to flash, so I'd top the reservoir off.

If I did a pad change I'd actually open the nipples and push the old fluid out that way rather than push dirty **** back up the lines.
The fluid in the calipers cops the heat cycles and oxidation so it's not a bad idea to purge it when changing pads if you aren't doing a full fluid change.

That works. My reply was to those who feel the need to draw fluid out of the cylinder, but if you hadn't topped it up you wouldn't have needed to.

I'm curious, just what dirty **** do you think got into your sealed hydraulic system? Heat transfers throughout the system, and actually very little is transferred to the hydraulics anyway. Designers have had a whole century to work this stuff out.

Ask any brake mechanic and they will tell you that topping up is bad practice.

I used to work on a rally car. We used to mark the reservoir with a texta, for worn pads and new pads. It was a simple way to see if the pads had worn enough to change them. Not a big budget concern, you understand. However, I still do this on some cars, as it makes sense. Brake fluid does not evaporate, it doesn't go away. If you are losing fluid then you have a problem. Mere dropping of levels due to pad wear is nothing to worry about.

If your car flashed it's warning light then that's a fault in your car. It's a Land Rover, after all. I have never seen the brake warning light come on in any of my cars, except when there has been a fault. Low fluid due to pad wear is not a fault. Perhaps you needed to have yours checked as well.

All that said, I like to change brake fluid when I get the chance. The mineral type attracts water, which isn't good ( water boils at way lower temps than brake fluid, and then there's the corrosion..). Pretty sure the synth sort doesn't, but old habits die hard.

Originally Posted by Andrew D

A goose wrote it.

Gee, thanks for that insight. I am now aware that at least one utter fool read it. I am so glad that you are a contributor to a forum where most people get along and share ideas. You'll fit right in.

Oh, and thanks for your intelligent rebuttal. All new ideas are welcome. Do you have any?

Originally Posted by johntins

That works. My reply was to those who feel the need to draw fluid out of the cylinder, but if you hadn't topped it up you wouldn't have needed to.

I'm curious, just what dirty **** do you think got into your sealed hydraulic system? Heat transfers throughout the system, and actually very little is transferred to the hydraulics anyway. Designers have had a whole century to work this stuff out.

Ask any brake mechanic and they will tell you that topping up is bad practice.

I used to work on a rally car. We used to mark the reservoir with a texta, for worn pads and new pads. It was a simple way to see if the pads had worn enough to change them. Not a big budget concern, you understand. However, I still do this on some cars, as it makes sense. Brake fluid does not evaporate, it doesn't go away. If you are losing fluid then you have a problem. Mere dropping of levels due to pad wear is nothing to worry about.

If your car flashed it's warning light then that's a fault in your car. It's a Land Rover, after all. I have never seen the brake warning light come on in any of my cars, except when there has been a fault. Low fluid due to pad wear is not a fault. Perhaps you needed to have yours checked as well.

All that said, I like to change brake fluid when I get the chance. The mineral type attracts water, which isn't good ( water boils at way lower temps than brake fluid, and then there's the corrosion..). Pretty sure the synth sort doesn't, but old habits die hard.

And I worked for several years professionally as a mechanic and setup engineer on race cars, but what's that got to do with anything?

Except we'd bleed the brakes every meeting, as in a full fluid flush as glycol ether brake fluid is extremely hygroscopic and the heat/cool cycles draw moisture into the fluid.

Buckets of heat are directly transferred into the fluid through the pad backing plates and pistons, its how the fluid degrades.

It's also why things like ceramic pistons have been used, to reduce heat transfer and it's why high boiling point fluids were developed.
The boilng point drops badly with moisture absorption, and this leads to localised boiling and creates a soft pedal, microscopic bubbles forming and creating a longer pedal.

AFAIK only older Citroen's and maybe some RR's used mineral oil as brake fluid.

DOT3, DOT4, Super DOT4 and DOT5.1 are glycol ether based and extremely hygroscopic.
The general recommendation is a full flush every two/three years to maintain a reasonable boiling point and reduce corrosion.

Originally Posted by rick130

And I worked for several years professionally as a mechanic and setup engineer on race cars, but what's that got to do with anything?

In your case, nothing. I fail to see the relevance to my point. I was merely pointing out the correlation between pad wear and fluid levels, as, having read many of your posts, I'm sure you are aware.

Originally Posted by rick130

Buckets of heat are directly transferred into the fluid through the pad backing plates and pistons, its how the fluid degrades.

It's also why things like ceramic pistons have been used, to reduce heat transfer and it's why high boiling point fluids were developed.

Sure, and perhaps bringing the rally car allusion into the discussion wasn't helpful. We're talking about a Disco here, a daily driver, and my rally stuff and your race stuff are not really on topic.

Originally Posted by rick130

The boilng point drops badly with moisture absorption, and this leads to localised boiling and creates a soft pedal, microscopic bubbles forming and creating a longer pedal.

Err, I already said that..

So, tell me, are you in the "OH! WOW! OMG!! MY BRAKE FLUID IS LOW!" camp or not?? If you are, tell me why.

Originally Posted by johntins

So, tell me, are you in the "OH! WOW! OMG!! MY BRAKE FLUID IS LOW!" camp or not?? If you are, tell me why.

I'm in the "the low brake fluid alarm is ****ing me off even though the level has only dropped 1/4" so I top it off the M/C" camp.

Originally Posted by rick130

AFAIK only older Citroen's and maybe some RR's used mineral oil as brake fluid.

DOT3, DOT4, Super DOT4 and DOT5.1 are glycol ether based and extremely hygroscopic.
The general recommendation is a full flush every two/three years to maintain a reasonable boiling point and reduce corrosion.

How does that contribute to the discussion? Understanding the car, the fluid needs of the car should have gone notwhithstanding. Golly, did I get that wrong?? I thought we were talking with people who had an idea. Mineral brake fluids were in use until a very few years ago. Many people on this forum are probably still using them. Sure, your unusual French fetish will change your perspective. I have an Italian one which changes mine.

What I find odd in all of this is that NOBODY has been able to refute my position, apart from the abuse from someone from "Sydney". Not one. Come on guys, am I wrong? Lotsa mechs out there, did I screw up? Or, as I have laid out, am I right? Do not top up brake fluid. I'd like an answer from the guys who do it for a living.