Yes, I'm aware of that fact. As the second can was unopened.

 

^This. Just did mine while swapping pads and lines and there was about 1/4L left in the bleeder bottle when it was all said and done (and I feel like I bled out a good amount of fresh fluid towards the end).

2 pints of RBF600 and a Motiv Power Bleeder and you're good to go!

Pro tip: although not 100% necessary, try to suck out as much of the old fluid from the reservoir before hooking up the power bleeder to avoid pushing old (potentially contaminated) fluid through the system. I used an old turkey baster, but I'm sure there is an actual "tool'" for doing that.

 

Sweet, thanks for the intel. Having never actually done fluid before, I for some reason had visions of needing gallons of the stuff.

 

Yeah, no worries. The power bleeder is clutch cause you can just hook it up, set the pressure (~10psi), and go around to each caliper (in proper order) and crack each bleeder screw until the fluid looks clear/newish and has no bubbles. Although also not 100% necessary, I like to lightly tap around on each caliper with a wooden mallet (or the end of a screwdriver) after I've cracked the bleeder screw to help dislodge any stuck bubbles. And then before the last round or two of bleeds, I also like to fire up the car and pump the brakes a few times to prime the system and make sure the pedal is feeling firm.

Oh, and make sure you clean up any fluid drips/spills on the car really well cause brake fluid is corrosive AF.

 

Also brake fluid will strip the finish off your paint in a heart beat.

 

Yup, think I said that

 

When you say the "last round or two of bleeds" are you meaning to say you bleed each caliper more than 1x in your routine, or are you saying when you've arrived at the last caliper, presumably front left for LHD, you use the pedal and pump the brakes?
Just curious. I hadn't seen anyone do more than one round.

 

Yes. I usually make 3 or 4 rounds to try and make sure I get a good flush of the entire system and at each caliper. The first round is usually the longest bleed and goes until I start to see a change in fluid color/clarity (makes it a bit tricky if you don't interchange with ATE SuperBlue, but you can generally tell when you're hitting "new" stuff) and then the following rounds are just me being OCD to use up the rest of the fluid and make absolutely sure there aren't any air bubbles left. If there isn't much left in the power bleeder when you're done, and you know you've spent a good amount of time at each caliper; then it's just math and can rest assured it's all new fluid in there.

Not sure if that's ASE-certified, but I've always done my flushes that way and never once had a soft pedal. Oh, and make sure you use proper bleed sequence too (clockwise starting at the furthest bleeder from the reservoir).

GL

 

^ That’s why you get 2 liters. I always put two in the tank as I’ve definitely used closer to 1.1 when I’m done rounds. You don’t want that tank going dry.

It’s time consuming but the only way to get the flush complete. I do it yearly, with smaller bleeds before track days.

 

This is the bleeding sequence for the C63.

 

It's fine if the pressure tank goes dry. You just don't want the MC reservoir to get below the min fill mark.

 

I get that. I meant if the tank goes dry and you’re not paying attention because you’re bleeding calipers, the MC can easily go dry.

I just put 2L in as default out of an abundance of caution and for convenience.

 

Don't you use SRF? Pissin' liquid gold down the toilet brah...

 

You’re right I misspoke. I put 1.5L in there and save the rest if I’m going to use it relatively soon. It’s been awhile, I’m due.

 

I was gonna start hanging out around your trash cans at night to see if you tossed any Benjamins that were too wrinkly for your wallet.

 

There is more to this, especially to older, low mileage cars:

The bleeder screws are a quite high grade steel, while the body of the caliper is Aluminum.
These are known in Material Science as dissimilar materials. They are subject to electrochemical corrosion known as Galvanic corrosion.

In the Japanese car world, as those cars have started to be equipped with Brembo brakes since around 2002, it is often found that someone has saved their car, having ridiculously low miles, or someone imported Brembo brake calipers for this or that car from a Japan junkyard, (where word has it, cars arrive with say 10,000 Miles, due to Japanese car taxation regulation) then the owner opens the bleeders of the calipers, only to take out at the same time the Aluminum threads in the caliper.

Now if you were to open the bleeders and stare at Aluminum Threads on the Steel Bleeders, here's the by-the-book procedure:
You can't buy just one new caliper, although the dealer will sell you one, I think. If you just buy one caliper, you will NEVER get matching brake torque on the respective axle. The car will pull under braking. I know, because in the 90's there was a special inspection in Eu for imported vehicles and they had a braking dyno requirement, there were cars that failed, failed, failed.... until people checked caliper status and discoverted one new one old on the same axle.
So you'd have to buy TWO calipers.

Now, knowing all of the above, what would the dealer do, if THEY while doing the brake flush take out some threads from inside the caliper.....
Would they take the high road, admit guilt and give you two new calipers?
What if only ... some threads get stripped?


See, the Mercedes often-brake-flush requirement sneekyly hides the requirement to turn those bleeders every year or so...

I'll tell you what though, I won't shut those bleeders without a torque wrench..

 

Although the caliper paint seems immune - I rinsed them off with water just in case, but getting brake fluid on them doesn’t seem to harm the red paint.

 

There are some great points in here, somewhere; but I’m having trouble following... try to separate your thoughts to 1 per post.
Now let’s see:
- are you saying that you torque the bleeder bolts? I don’t think that’s a good idea, especially since as you point out, then thread into aluminium, which is softer.
I don’t follow the rest - are you also saying that both calipers need to be replaced if one goes bad? Not sure I agree: why would both of the old calipers change braking force at the same time? There’s no reason why they would stay balanced.

 

Let me try to be more clear:

1. About torque:
What does a bleeder screw do?
When it is shut, it prevents fluid from seeping out and air from coming in ...
a) What would too little fastening torque applied to it do?
Put you in danger, as in theory it could even hydraulically auto-unscrew, under the 1000 PSI brake fluid pressure ram that it feels when you slam on the brakes. The correct fastening torque prevents this from happening.

b) What would too much fastening torque do?
With a brand new caliper fitted with a brand new screw, there is some tolerance to where a well versed mechanic that has done a lot of brake bleeding in the past can guess at the fastening torque within reason and not strip threads.
Let's review what happens with an old caliper:
Resistive force caused by friction (fastening IS friction) features a spike in value, as you apply loosening force with a wrench. Torque wrenches can be used to loosen too and useful information can be measured. Normally, what opposes your wrench as you loosen the bleeder is the friction caused by tightening torque applied when the car was new. The Aluminum threads are designed to whitstand this without a doubt.
But after 10 years, it's that + the onset of galvanic seizing.

With a caliper that has seen a couple miles (say 8000), but, most importantly has not had the bleeder removed for 10 years, even when you successfully remove the bleeder screw, it has seen as above, quite the extra stressing force, when being removed the first time since 10 years ago.
I would not attempt a guess at the fastening force/torque, without the assistance of a torque wrench. This assembly will not tolerate an over tightening error anymore.
It would invite aluminum threads stripping, as the weak-to-begin-with Aluminum threads are stressed by over-tightening them even a little bit.
If your car saw brake flushing as per the MB schedule, then fine. If it has not, be careful.

c) what to do then?
Open the WIS and read the factory recommended bleeder screw tightening torque. Grab the torque wrench and fasten it to that exact torque.

2. About used brake calipers:

As time goes by, calipers which are of the same age and have operated together on the same car, wear even. Calipers wear too, just like brake pads and rotors. Their wear doesn't require their replacement, as the brake booster provides sufficient amplification to the braking force, to where the extra stepping on the pedal force increase is not felt. To feel it, drive back to back a 5000 Mile 2014 C63 and a 90 000 Mile C 63 (with new pads+rotors). There will be a mild felt difference.
Here's how this translates:
The brake boosting system takes your increase of stepping on the pedal force and amplifies it across all 4 calipers by a factor of 10. The caliper itself provides a clamping force which is amplified when compared with your foot stepping force an another factor of 10 (I don't give these numbers as absolute truth, but as an idea, feel free to correct me about them).
While the hydraulic system is equalized and the amplification is as designed, even, the local amplification of the caliper varies from new to old.
By example, let's say the old caliper still provides an amplification of 8 times. You pair it with a new caliper, you'll have uneven braking force/torque and the car pulls under braking.
Pulling under braking can be of two kinds:
The bad one:
Step on the pedal and hold on to your pants
The one that backstabs you:
Step on the pedal and all seems fine, however one day on ice you tap lightly and it brakes totally uneven. To prevent this from happening, braking dynos have been invented. They feel braking bias that you don't normally feel. Remember the dyno mode? What if that is engine OR brake dyno mode?

If you're asking me what would ABS and traction control do, when faced with uneven braking bias? I don't know. Maybe they can automatically account for this to where it doesn't matter (they act on individual wheels these days), maybe they're pre-programmed to assume that no braking bias exists.

Me personally, I would not hang my hat on them compensating seamlessly though. the ABS works by momentarily interrupting braking force, so if one wheel has more than the other (for instance Front left and Front right), then braking hard with these will cause one to lock up and ABS to interrupt and the car advancing further under braking, as compared with both wheels getting the same braking force, maybe I'm wrong.

 

What wear? Piston seals? Increased flex?

 

Makes sense @Vladds, thank you for the additional details!

It would be tough to get a torque wrench in there for the bleeder bolts, but I can see how if it hasn’t been done every two years, the bolts are more susceptible to stripping - either the threads or the head can become rounded — both are very tricky to solve without a new caliper!

 

I can get into the details of what wear.

Two things:
1. I'll be honest and say that while I saw cars fail the brake dyno and pass with pair of caliper replacements, I can't give my following explanation as factual. This is ... my explanation, my theory as of why they wear.

2. With this, the overall length of my remarks increases, don't hold it against me that this is now a long conversation


So, the caliper local amplification has some components:
1. The mechanical advantage, caused by the position of the caliper relative to the center of the wheel, and this doesn't change obviously.
2. the relationship between the diameter of the braking piston and the pressure provided to the piston by the hydraulic system

To these two, I add the resistance to movement of the piston, due to the seals. Again, my theory, this is not factual.
I have experienced several times in several cars complete braking system lock up, due to swollen master cylinder seals. Swollen seals can stop your car locked-up tight. And they're just ... pieces of rubber.

Now for a little bit of fact:
Rubber swells when immersed in oil. There are rates and different rubbers have different mandated rates, but they exist nevertheless.

Back to theory:
IF we admit that a small piece of rubber can cause your car to not move wheels even when hooked to the winch of a flatbed, just because it swelled a little bit, then why cant we admit that there's a difference in hydraulic resistance between a new and used caliper?

Some other, more vague theories:
looking at Brembo calipers and their rebuild kits, there are some shims. These shims (or anvils are they called?) are responsible for keeping the brake pad positioned during braking. If a pad can tilt under braking, the overall force the caliper provides can maybe change?
For multiple piston calipers, did you see sometimes how the rotors wear more inside than outside? The cause of this can be other than the piston seals. Maybe the inner side sees more heat and in time starts to retract slower? Maybe flex, as you say?

 

You are thinking WAY too low level man.

As long as there's no piston binding and the volume is the same between each monoblock caliper your bias will be fine.

 

@Vladds - can you please post up the correct bleeder screw torque values for sake of posterity, I’m about to replace mine while rebuilding my calipers and would be handy to know. Thanks

 

It was 18 Newton meters for the front and rear bleeders, then follow up notes they revised the rear bleeders to 17 Nm, front to 14 Nm.