One Question on Brake Replacement

Jkb242

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May 19, 2019
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It came across an article that suggested cracking the bleed valve before compressing the caliper when fitting new pads. The concern seem to be to avoid pushing fluid backwards and damaging the ABS check valve. Should this be done or does collapsing the piston carry this risk?

Also at one point in the past I found the rear or inside pad having far greater material remaining.compared to the outer pad. Is this normal or does it point to a caliper issue?

Thanks!!
 

Redbeard

Member
Jan 26, 2013
3,466
It came across an article that suggested cracking the bleed valve before compressing the caliper when fitting new pads. The concern seem to be to avoid pushing fluid backwards and damaging the ABS check valve. Should this be done or does collapsing the piston carry this risk?
I'm unsure if it damages the ABS valve, but if one keeps the master cylinder "reasonably full" the fluid from the caliper will over fill the master cylinder reservoir and flow all over the engine compartment if the bleed valve is not open. (ask me why I know this :sadcry: ). Just to be safe crack open the bleed valve while pressing in the caliper for the new pad. Plus you are pushing old fluid up into the master cylinder instead of keeping fresh fluid flowing down to the calipers.

On the other question of one pad wearing more than the other: This suggests the caliper is not fully retracting after force of applying the brakes is released. Often this is due to dirt and trash being found between the "sliding" parts of the caliper especially if proper lubricant was installed there. I have also seen this happen when the pads become thin and the piston becomes "non concentric" (being extended well out from the cylinder and hence not given much guidance) and at that point doesn't like to be pushed out or in much causing drag on the break pad.
 

Jkb242

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May 19, 2019
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Thanks Redbeard. Taking an extra moment to bleed off the caliper while collapsing the piston is indeed something I should do. The uneven wear differences sounds like an easy fix which proper cleaning and lubing should address as you recommended.
 

mrrsm

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My two favorite instructional videos on the two issues of Universal Disc Brake Rebuilds and How to Properly Bleed Brakes and avoid getting Air in the Lines are THESE:

Please disregard the YouTube Title on this First One...Trust me...What Eric "O" from South Main Auto knows...EVERYONE should Know about these issues. Right at 20 Minutes into this YT Video, he explains how "Rust Jacking"can cause asymmetric Brake Pad Wear... and reveals The Secrets of How to Prevent the Problem:


And ChrisFix shows the proper technique for Brake Bleeding whenever you are All By Your Lonesome:

 
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Jkb242

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May 19, 2019
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Ha great, I know these two well! Thanks
 
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Mooseman

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Dec 4, 2011
25,262
Ottawa, ON
This has been a subject of debate on and off. Sometime in the distant past, somebody thought it was a good idea to crack open the bleeder while pushing the pistons back in even though THERE IS NO MANUAL THAT EVER SAID TO DO THIS except possibly for the most recent vehicles.

That being said, it's not a bad idea to do it. You do replace a little of the old fluid with new fluid in doing so. However I've never done it and IIRC, Eric O. Doesn't neither. This is a PITA if in the rust belt and the bleeder is stuck or breaks off.
 

Chickenhawk

Member
Dec 6, 2011
779
I side with the Moose. Unless your brake fluid is overfilling your reservoir - which shouldn't happen unless owners top up fluid and the pads are VERY worn down - I wouldn't worry about it. On the other hand, it is a good idea to bleed and change all the brake fluid every few years. It gets rid of water, and especially prevents rusting bleeders, as my friend mentioned.

As for uneven wear, I have never seen a Trailblazer yet that didn't wear more on one side than the other. Unless it's excessive, I wouldn't worry about it. Just make sure you properly lube all the sliding parts on the new pads.
 

Reprise

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Sometime in the distant past, somebody thought it was a good idea to crack open the bleeder while pushing the pistons back in even though THERE IS NO MANUAL THAT EVER SAID TO DO THIS except possibly for the most recent vehicles.

The only way I'd even consider doing this is if I'd already installed 1-way bleeders (aka: 'speed bleeders'). Doing this on conventional bleeders (and just leaving them open while you push the piston(s) back in) is going to introduce air into the system, which will have to be bled out.
If you don't do it before putting the vehicle in service, it's going to be more trouble, later (because the air doesn't just stay at the end of the line). At that point, you may as well replace all the fluid, anyway.

Knowing the way lazy Americans take care of their cars (which means, 'no one on this board'... lol), as long as the vehicle's brakes operate when pushed, they're not adding fluid as the pads wear. They're lucky to keep their oil changed. So there's probably plenty of room in the reservoir, on average, to handle the (now extra) fluid displaced by reseating the piston. The exception is when you purchase the vehicle used; then you don't know what's been done (and most of us would prolly replace all the fluid at that point, just on principle.)

Now... that said... would I loosen up the cap on the reservoir, while pushing the piston back in the seat? Yeah, I think that's good practice. I try and do that, myself. That also allows me to note how high the fluid is in the reservoir, too.

Finally, remember that the 'full' mark on the reservoir isn't at the tippy-top - it's at a lower point, where there's still room for fluid. So under those 'normal' circumstances (no one adding fluid as the old pads wore down), it *shouldn't* overflow the reservoir. Especially from doing one caliper at a time, and then checking the level.

I keep a $1 store turkey baster in the garage for removing any excess (and also for removing as much old fluid as I can, before starting a fluid replacement.) No sense in sending it out bit by bit through the caliper, when I can easily suck it out at the other end & replace with clean. Shortens the time to get the remaining old stuff out of the lines.
 

mrrsm

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This Classroom White Board Basic layout of Fluid Dynamics is valid for ANY Hydraulic System because The Principles of Hydraulics always apply, whether the Fluid in Question is Water, Oil and even Atmosphere itself. They all must follow the theories laid down by Bernoulli (a Man who's work was sadly later stolen by his own jealous father... who took the Academic Credit for his son's Ideas.) What is important here though is that it makes a perfect and instructive Backcloth for showing the Scientific basis for why @Mooseman 's response *** is the Correct and Best Explanation on this matter:


THIS Instructional Video is about Hydraulics Math and is LOADED with baseline information on how these systems function. This added knowledge will be useful in all circumstances of trying to fix everything from Automotive Brakes to Broken Floor Jacks:




*** Not that he needs any help...
 
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