Depressing caliper pistons

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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.
 
Why is that a downside?

Admittedly a full bleed would be better, but a slight bleed seems better than none.
 
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.
 
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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.
 
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.
 
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.
 
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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.
 
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.
 
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.
 
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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.
 
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.
 
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.
 
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.
 
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I've taken off calipers and dumped the old fluid out before. If the brake fluids rarely (if ever) been changed, the brake fluid that comes out is really dark and full of sediment. It's kind of thick too. Not Lucas thick, but thicker than brake fluid.
 
It's not a completely sealed system as the Master Cylinder has to be vented or it will not work under a vacuum that is caused as the pads wear and the fluid level drops. The caliper obviously sees the most heat which is what darkens the fluid. 10 year old fluid will be water contaminated through and through which makes it corrosive.
 
Originally Posted By: SHOZ
It's not a completely sealed system as the Master Cylinder has to be vented or it will not work under a vacuum that is caused as the pads wear and the fluid level drops. The caliper obviously sees the most heat which is what darkens the fluid. 10 year old fluid will be water contaminated through and through which makes it corrosive.


Brake fluid is corrosive, it's a very good paint stripper.
However it is hygroscopic, that is, it absorbs water. Normally atmospheric humudity via the master cylinder vent. However water needs oxygen to cause rust, which if present in a braking system would be rapidly obvious.
As has been said, the calipers get very hot, and coupled with the pots passing over the seals and possibly introducing contaminants will cause deterioration inside the caliper.
So bleeding the calipers when changing the pads seems a sensible thing to do, when you top up afterwards it will help improve the quality of the brake fluid in the system as a whole, just as partially replacing automatic transmission fluid as recommended in the service schedules of most cars and trucks

Claud.
 
Keep in mind that the master cylinder is vented using a bellows. The fluid is NOT in direct contact with the atmosphere. The fluid draws the bellows down. The side of the bellows opposite the fluid is vented through a pinhole in the cap.
 
Originally Posted By: emmett442
Keep in mind that the master cylinder is vented using a bellows. The fluid is NOT in direct contact with the atmosphere. The fluid draws the bellows down. The side of the bellows opposite the fluid is vented through a pinhole in the cap.
Have you ever heard any air enter when removing the cap?
 
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