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#4757520 - 05/15/18 03:40 AM Depressing caliper pistons
mjoekingz28 Offline


Registered: 11/21/07
Posts: 3996
Loc: United States of America
I was wondering if it is good practice to open the brake bleed screw before compressing the piston while installing new pads.

It seems it would expel the fluid you cannot remove with a flush (as I have read on here) and also make the piston retract easier as it doesn't have to push fluid into the reservoir.

Downside though is a slight bleed is required.

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#4757548 - 05/15/18 05:46 AM Re: Depressing caliper pistons [Re: mjoekingz28]
Ducked Offline


Registered: 10/25/12
Posts: 4465
Loc: Taiwan
Why is that a downside?

Admittedly a full bleed would be better, but a slight bleed seems better than none.

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#4757549 - 05/15/18 05:47 AM Re: Depressing caliper pistons [Re: mjoekingz28]
Char Baby Offline


Registered: 05/25/05
Posts: 10175
Loc: ROCHESTER, NY
IMO, this is the best way to depress the pistons as it doesn't push back dirty fluid into the system. Many techs don't do this and may not have any issues either way. But, I like opening the bleeder.

However, first try pushing back the piston(just a little) without opening the bleeder. If the piston doesn't move or is difficult to push back, you may have a slightly blocked hydraulic line/brake hose. If the piston is easy to push bake w/o the open bleeder, then proceed to open the bleeder and finish pushing back the piston. This is only for testing and something I like to do.

Continue to bleed the whole system as good PM.


Edited by Char Baby (05/15/18 05:48 AM)
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#4757583 - 05/15/18 06:58 AM Re: Depressing caliper pistons [Re: mjoekingz28]
A_Harman Offline


Registered: 10/01/10
Posts: 6972
Loc: Michigan
I always bleed the caliper until clean fluid comes out prior to pushing the pistons back in. It just keeps junk from getting up the brake pipes.
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#4757613 - 05/15/18 07:53 AM Re: Depressing caliper pistons [Re: mjoekingz28]
46Harry Offline


Registered: 01/08/12
Posts: 414
Loc: hawthorne, Ca.
I always open the bleeder screw when depressing the caliper piston as it keeps the trash that has built up from being pushed back into the hydraulic system.

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#4757649 - 05/15/18 08:34 AM Re: Depressing caliper pistons [Re: mjoekingz28]
ZZman Offline


Registered: 03/17/08
Posts: 5902
Loc: Michigan
I assume opening the bleeder makes it super easy to push the piston in?
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#4757658 - 05/15/18 08:43 AM Re: Depressing caliper pistons [Re: mjoekingz28]
ARCOgraphite Offline


Registered: 05/17/09
Posts: 12117
Loc: N.H, U.S.A.
Just use a big flat blade screwdriver and wedge it between the rotor and floating side of the caliper and lever compress. This is for the NOT RECOMMENDED quick pad slap. No fancy Clamps required.

Now, It Better and more Professional to do a complete flush all 4 corners.

If I was working on a nice car I liked; I would bleed when compressing through a rubber hose into a waste can of DOT 3. I HATE brake fluid - always have. The suff down at the business end is watery, corrosive and just plain bad.


Edited by ARCOgraphite (05/15/18 08:47 AM)
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#4757684 - 05/15/18 09:01 AM Re: Depressing caliper pistons [Re: mjoekingz28]
maxdustington Offline


Registered: 01/21/17
Posts: 1014
Loc: Toronna
If you are going to bleed anyway, what difference does it make if some dirty fluid gets pushed up the brake line? Like a foot or two?
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#4757737 - 05/15/18 09:30 AM Re: Depressing caliper pistons [Re: mjoekingz28]
KrisZ Offline


Registered: 12/23/06
Posts: 7555
Loc: Toronto, Canada
I don't do this.
My rationale is that, if the junk is in the piston, contacting the seals, etc. and not having negative effects, what harm will it do if it gets pushed back into the steel brake lines? It's not going to go all the way back to the master cylinder.

Also, I have never seen the "junk" people mention. Maybe few "floaters" when looking directly at a strong source of light. Otherwise the brake fluid in my vehicles looks like aged whiskey.
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#4757744 - 05/15/18 09:38 AM Re: Depressing caliper pistons [Re: mjoekingz28]
emmett442 Offline


Registered: 12/07/16
Posts: 186
Loc: Wisconsin
I still think this is all a myth.

Where is the "dirt" and "junk" in the fluid coming from? The fluid in filling the caliper bore is the same fluid that's in the lines and master cyl. It's a sealed system. If air and fluid can't get passed the caliper piston, how's dirt and junk going to? There's no reason for the fluid in the caliper to be any more dirty than the rest of the system.

I prefer to compress with the bleeder closed so I can "feel" the condition of the calipers and the rest of the system. It's harder to notice a sticking caliper piston, collapsing brake hose, etc if the bleeder is open.

I do crack the bleeder loose afterwards, but that's to enure that the bleeder itself stays free and doesn't sieze in the caliper. Every time I check or replace the brakes, I remove the bleeder, anti sneeze, and reinstall.
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#4757783 - 05/15/18 10:23 AM Re: Depressing caliper pistons [Re: emmett442]
JTK Offline


Registered: 08/14/03
Posts: 10689
Loc: Buffalo, NY
I've never seen a used caliper bore without junk in it. I dunno if it's from the piston moving in/out, seal material, or the fact it's the closest area to the outside world.

Problem is, the heavy bits are going to lay and stay in the bottom no matter what you do. The bleed screw is up top. If you're confident the bleeder is going to move and re-seat, I'm all for cracking it prior to compressing the piston.

I'm hit/miss in doing this. Depends how much time I've got and how in-depth I wanna go.


Edited by JTK (05/15/18 10:23 AM)
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#4757918 - 05/15/18 01:07 PM Re: Depressing caliper pistons [Re: mjoekingz28]
andyd Offline


Registered: 09/25/04
Posts: 7431
Loc: Marshfield , MA
I've the hole so packed with crud, I had to use a needle to clear it. BF attracts water, water and air cause corrosion in the cylinder bores when the water boils off.
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#4758355 - 05/16/18 12:39 AM Re: Depressing caliper pistons [Re: mjoekingz28]
NoNameJoe Offline


Registered: 06/03/15
Posts: 459
Loc: New York
I too have no idea where the crud comes from but I've seen it. Sometimes after doing a brake fluid change at all four wheels I'll find the jar has some kind of sediment on the bottom. The jar is capped except for a hole at the top for the tube to go through so it must have come from the fluid.

There's also some kind of sediment inside the reservoir sometimes.

As for whether I open the bleeder when I depress the piston, I like to change the fluid at the same time I do anything that involves depressing the piston. I usually change the fluid first, then depress the piston without opening the bleeder since I know I have clean fluid. I have depressed the piston with the bleeder open sometimes too though.

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#4758418 - 05/16/18 05:54 AM Re: Depressing caliper pistons [Re: ARCOgraphite]
Ducked Offline


Registered: 10/25/12
Posts: 4465
Loc: Taiwan
Originally Posted By: ARCOgraphite
J I HATE brake fluid - always have. The suff down at the business end is watery, corrosive and just plain bad.


Then someone isn't changing it often enough. Maybe me.

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#4758428 - 05/16/18 06:13 AM Re: Depressing caliper pistons [Re: emmett442]
Ducked Offline


Registered: 10/25/12
Posts: 4465
Loc: Taiwan
Originally Posted By: emmett442


I still think this is all a myth

Where is the "dirt" and "junk" in the fluid coming from?........It's a sealed system.......................................................... Every time I check or replace the brakes, I remove the bleeder, anti sneeze, and reinstall.


Maybe someone is putting anti-seize in it? Y'know, oil-based, rubber incompatible stuff?

That (fairly slim admittedly) possibility aside, if it was a completely sealed system, you'd never have to change your brake fluid.

You do.

So it isn't.

Given that it isn't, its a fair bet that it isn't uniformly permeable, so there may be local concentrations of water. I've seen it stated that water gets in via hose permeability. If that's true, it perhaps also gets in through the piston seal. This would tend to produce a higher concentration of water in and behind the pistons.

In addition, there is movement and friction in the pistons (and master cylinder) which will tend to put debris from seal and bore wear into the fluid.

I've never seen an analysis of the distribution of water and debris in the system, but anecdotally the first bit of bleed tends to be darker, which tends to confirm expectation.

So if its a myth, its at least an unusually reasonable one, as myths go.


Edited by Ducked (05/16/18 06:21 AM)

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