Master Cylinder Upgrade for Big Brake Kits Thread

[rubicant5x]

So in planning for my upgrade of my front brakes to the RS5 brakes, I wanted to upgrade to the RS5 master cylinder for better brake pedal feel and overall best practices. I did some digging with the help of HSTuning to get the following OE piston sizes for other Audi vehicles:

The RS5 is 26.99mm
The S8 Armored Edition is 28.57
The A4/S5/S4/S5/S6/S7/RS7 is 25.4

Bigger piston size = better braking pedal pressure, etc.

The problem I have found is the external housing for all the above are different so it's not a matter of an easy swap but some Frankenstein type stuff.

Here is what I have come up with so far.

The B8 and B8.5 S4 have different master cylinder part#'s. According to ECS, they go as follows:

B8: 8K0611021B

B8.5: 4G2611021A


You can see the B8.5 has a larger lip and is shaped a bit different than the B8 is.

Now, here is the B8 RS5:
8T0611021


From these images, you can see the RS5 looks almost identical to the B8 master cylinder but with a bigger piston. I've had the RS5 master in my hand and can confirm the picture for it is accurate.

From my RS5 Brakes thread, http://www.audizine.com/forum/showth...Brakes-Upgrade we are thinking we might be able to swap the guts of the RS5 MC into the stock B8.5 MC housing to get the larger piston size but in the compatible housing. The other option is to install (if able) the B8 brake booster so the RS5 master will fit in the assembly.

I have no data on the S8 Armored edition master cylinder but it would be a good option if it can be made to work.

Even though this thread is geared towards getting more pedal pressure for the RS5 BBK, it can certainly work for any BBK upgrade (Porsche 19z, Brembo, etc) and would like to get a solution for everyone who wants bigger brakes but to retain the same/similar pressure of the stock setup. Not so much for feel but to ensure the brake pads wear evenly and avoid warping the rotors themselves. I will update both threads with progress but any contribution is greatly appreciated!

[SwankP3RF3ct10n]

So, if I understand this correctly, if you have a B8 S4, there is no problem swapping to the RS5 master cylinder, since the parts are almost identical externally. Right or wrong? I gather you had issues cuz you have a B8.5, is that right?

[rubicant5x]

So, if I understand this correctly, if you have a B8 S4, there is no problem swapping to the RS5 master cylinder, since the parts are almost identical externally. Right or wrong? I gather you had issues cuz you have a B8.5, is that right?

In theory, yes and yes, I have a 2014.

[Pawan2013]

this is big news for B8 guys

[Jroyalty7]

This is a good thread man, looking forward to see what you uncover! I was actually looking at grabbing the set you bought, just was a little slow on the trigger lol. Soon enough, soon enough ;)

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[speedy0339]

So we an put in the rs5 mc in a b8 s4 for better brake feel?

[rubicant5x]

So we an put in the rs5 mc in a b8 s4 for better brake feel?

Only when you have upgraded calipers like the RS5 or 19z BBK. After a BBK upgrade, the stock braking system (master cylinder) cannot drive the calipers as well as they can the stock setup.

[speedy0339]

I have a stock system with stoptech rotors and pads in the front and I have horrible brake feel :(

[FlyS4]

I was also searching around and came up on a info that A8 brake booster fits into the RS5 any idea if A8 master and booster are better than b 8.5?

[abamfo]

Has anyone upgraded to the 19z or Rs5 brakes with the stock B8.5 master cylinder?? Are they definitely not a more powerful unit than the b8

[rubicant5x]

Has anyone upgraded to the 19z or Rs5 brakes with the stock B8.5 master cylinder?? Are they definitely not a more powerful unit than the b8

Read my thread that is linked in the top of this one...

[blackfunk]

From my understanding in a properly designed BBK the caliper pistons are sized so that the overall size of the multi piston setup stays within the % of of the margin of error so a larger master cylinder is not needed. Essentially the volume of fluid in the line stays about the same but due to the fact the pistons are smaller they're more 'agile' allowing for better brake feel. I think I found it in one of the videos from these guys - https://www.youtube.com/watch?v=p8eDnkjL2V4 - they have too many for me to sift through ATM but I believe its mentioned in there. Either way they are an excellent resource for anyone interested in moving to a BBK - (in my best trump voice) - informative, very very informative. You wait and see. LOL

For me going bbk the most noticeable improvement was the improved brake feel. The ability to modulate the peddle at the threshold just before ABS kicks in. I was not able to do that with the stock system. Granted, different pads have a huge bearing on this non scientific test but for me the difference was huge.

[-leman-]

Aren't RS5 brake booster and ABS module different too? If it is I'd swap both for "overall best practices" as you say... If you swap the MC only but keep the OE booster (with presumably lower boost coefficient) you will get less brake pedal travel / firmer pedal (because bigger MC piston size) but higher force on the pedal required for the same braking torque. Who knows how it'll feel subjectively but why all the hassle when you can just take all RS5 parts which have been engineered to work together?

I personally just have the RS5 calipers on OE MC / booster / ABS module and I'm quite happy with the pedal feel...

[s4bw]

Aren't RS5 brake booster and ABS module different too? If it is I'd swap both for "overall best practices" as you say... If you swap the MC only but keep the OE booster (with presumably lower boost coefficient) you will get less brake pedal travel / firmer pedal (because bigger MC piston size) but higher force on the pedal required for the same braking torque. Who knows how it'll feel subjectively but why all the hassle when you can just take all RS5 parts which have been engineered to work together?

I personally just have the RS5 calipers on OE MC / booster / ABS module and I'm quite happy with the pedal feel...

Same setup here, I had ceramic pads to start with. Pedal feel was awful, went to HPS 5.0 and made a world of difference. OP I would start with pads first and maybe try bleeding them again before I changed the MC etc.

[blackfunk]

Agreed. Pads make the biggest difference for feel and it would be where I would start. Its also the cheapest place to start.

[rubicant5x]

I'm going to pick up the B8 brake booster and see if the RS5 MC will fit. The revision difference between the B8 and B8.5 part number is a letter difference at the end so I am hoping it will bolt up to my 2014. We shall see. Thanks for all the input.

[will13k7]

with a b8 s4 and a brembo gt bbk specifically made for a b8 s4, the modulation felt amazing, I wouldn't change it. I thought they sized everything to work well with the oem master cylinder.

taking a brake kit from one car and putting it on another, that's where I think you'll benefit from matching other parts. I have the 8V RS3 front brakes (same as the rs5) on my 8V S3, and the modulation isn't even close in comparison to what a brembo gt on a b8 s4 felt like, but still a worthy upgrade.

[rubicant5x]

with a b8 s4 and a brembo gt bbk specifically made for a b8 s4, the modulation felt amazing, I wouldn't change it. I thought they sized everything to work well with the oem master cylinder.

taking a brake kit from one car and putting it on another, that's where I think you'll benefit from matching other parts. I have the 8V RS3 front brakes (same as the rs5) on my 8V S3, and the modulation isn't even close in comparison to what a brembo gt on a b8 s4 felt like, but still a worthy upgrade.

I would have gone the Brembo GT route for my S4 but I couldn't pass up the RS5 calipers, rotors, pads, etc. All in all, with all the upgraded parts, (RS5 complete front and upgraded rear rotors and EBC RedStuff pads) it was a $2000 investment so I can't complain. Even with $700 in RS5/B8 parts, still a great upgrade for the price.

[will13k7]

... still a great upgrade for the price.

Better fitment with the RS5's too, the brembos are a tad too big, makes finding wheels a huge pain.

[FlyS4]

I just had an hour long conversation with a buddy of mine who is has multiple degrees, and knows thermal dynamics well. Basically he told me this: THere are 2 things that need to be met for this to be a properly working setup. 1. Proper supporting hardware to supply pressure to the oversized calipers, 2) heat management. First is accomplished by simply getting the RS5 stuff (master/booster). Now the heat management is tricky. S4 has no ducting for wind to chanel in for cooling. Extra braking energy will translate into extra heat without a modification to expel it. HAve you looked into a Macan scoop? would that possibly provide enough cooling to maintain the system within its heat range?

I have the kit sitting in my garage. I don't know if it's a good idea unless there is a solution to the cooling problem.

I hope these Macan shields would be enough because i do want to get this all to work right.

[rubicant5x]

I just had an hour long conversation with a buddy of mine who is has multiple degrees, and knows thermal dynamics well. Basically he told me this: THere are 2 things that need to be met for this to be a properly working setup. 1. Proper supporting hardware to supply pressure to the oversized calipers, 2) heat management. First is accomplished by simply getting the RS5 stuff (master/booster). Now the heat management is tricky. S4 has no ducting for wind to chanel in for cooling. Extra braking energy will translate into extra heat without a modification to expel it. HAve you looked into a Macan scoop? would that possibly provide enough cooling to maintain the system within its heat range?

I have the kit sitting in my garage. I don't know if it's a good idea unless there is a solution to the cooling problem.

I hope these Macan shields would be enough because i do want to get this all to work right.

Good stuff, man.

I just ordered a B8 brake booster, part# 8K0612107C (there is a "A" revision, also). I am heading over to the shop tomorrow to compare the B8 RS5 MC and the B8.5 S4 MC to see how the dimensions look if we have to swap guts. I am going to see if the RS5 will fit into the B8 Brake Booster first. Stay tuned.

[rlfletch]

I hope these Macan shields would be enough because i do want to get this all to work right.

Has anyone installed the Macan scoops on a B8.5? Do they work? Is the install easy? Part number and cost?

[FlyS4]

Has anyone installed the Macan scoops on a B8.5? Do they work? Is the install easy? Part number and cost?

Yes, they just clip on. The picture is of the scoop installed onto a B8.

Here is were you can buy them for 7.55. Don't forget clips.
https://www.ecstuning.com/Porsche-20...aking/Cooling/

[rubicant5x]

UPDATE -

My B8 booster came in and it's looking REALLY good. here is the RS5 MC inside the B8 booster





It's getting installed on January 3rd so I will report back.

[abamfo]

This is a fantastic thread, looking forward to seeing the results from the upgraded master cylinder... re. the cooling requirement, do you guys use the stock S4 dust/heat shields, the rs5 shields which have ducting in them or remove them altogether?

[yjypm]

Yes, they just clip on. The picture is of the scoop installed onto a B8.

Here is were you can buy them for 7.55. Don't forget clips.
https://www.ecstuning.com/Porsche-20...aking/Cooling/

So 2 ducts and 2 clamps on the website you linked are all the things needed?
Sounds like a very affordable upgrade.

[FlyS4]

So 2 ducts and 2 clamps on the website you linked are all the things needed?
Sounds like a very affordable upgrade.

Yes, those are all that is needed to help to channel the air towards the RS5 shield with hoes in it. Keep in mind there would be not much air to chanel as the steering tie rod it attaches to will not be in direct wind. I am most certain there is more modifications needed to get the air from up under into a wheel well blowing directly onto macan diffuser thingy...

[FlyS4]

Matt,

What is the difference between B8 and RS5 booster? Why not go with RS5 booster from the beginning?

[rubicant5x]

Matt,

What is the difference between B8 and RS5 booster? Why not go with RS5 booster from the beginning?

RS5 has an external vacuum pump, from what I am told because it's a high-revving, NA engine.

[ReDeViL]

Looking forward to further updates on this thread. I'm on the rs5 brakes, with the naca ducts and piping mods, would love to hear feedback on the rs5 brakes boosters.


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[DBFL]

Yes, those are all that is needed to help to channel the air towards the RS5 shield with hoes in it. Keep in mind there would be not much air to chanel as the steering tie rod it attaches to will not be in direct wind. I am most certain there is more modifications needed to get the air from up under into a wheel well blowing directly onto macan diffuser thingy...

Correct. I used naca ducts attached to the belly pan and 3" hose connected to a flange in the wheel well liner. This directs the air directly into the Macan ducts. There are a couple pictures of my setup in my build thread. Also, I used the dust shields off a Macan. The stock S4 shields would block the air. The RS5 pieces have holes to let some air through. I found that the Macan shields expose the rotor to the air the best while still shielding all of the rubber components from heat. And the Macan dust shields are a direct bolt on with no modification necessary.

[FlyS4]

Correct. I used naca ducts attached to the belly pan and 3" hose connected to a flange in the wheel well liner. This directs the air directly into the Macan ducts. There are a couple pictures of my setup in my build thread. Also, I used the dust shields off a Macan. The stock S4 shields would block the air. The RS5 pieces have holes to let some air through. I found that the Macan shields expose the rotor to the air the best while still shielding all of the rubber components from heat. And the Macan dust shields are a direct bolt on with no modification necessary.

I already have the RS5 shields. I am not sure which direction I will be going as of yet. Just ordered MACAN stuff from Rector in cali. Was all under 50 shipped. I also ordered Q5 Calipers. I want to try those 1st before I put on RS5 brakes.

Q5 stops in 109 feet. Test was done by the same people who reviewed the S4 and got the same stopping distance out of S4. I would imagine Q5 calipers would make S4 stop better than 109 feet. I think I should be happy with Q5 calipers. Those plus macan shields and ducts plus nacas will be an upgrade over stock garbage A4/S4 brakes.

[rlfletch]

So I ordered the Macan Air scoops and clips. Does anyone have the part numbers for the correct Macan dust shields? Is 95B615311B and 95B615312B correct?

[FlyS4]

So I ordered the Macan Air scoops and clips. Does anyone have the part numbers for the correct Macan dust shields? Is 95B615311B and 95B615312B correct?

Those are the same I ordered. Got it off someone who did macan swap.Saw MAcan in person today, caliper looked pretty massive. OEM rotors are not that great though, one piece.

[FlyS4]

RS5 has an external vacuum pump, from what I am told because it's a high-revving, NA engine.

That shouldnt effect the operation of the Booster. I would imagine "Audi" regulates the vacuum evenly across many platforms.

[rubicant5x]

That shouldnt effect the operation of the Booster. I would imagine "Audi" regulates the vacuum evenly across many platforms.

Sure it does. If you have an external source for vacuum instead of through the system, its going to affect how the brake booster works. How is the booster supposed to operate in the absence of the external pump. The master cylinder is what is actually doing the work on the brakes. All the booster does is give the brake pedal more oomph so you don't have to smash the pedal for it to drive the master cylinder.

[FlyS4]

Sure it does. If you have an external source for vacuum instead of through the system, its going to affect how the brake booster works. How is the booster supposed to operate in the absence of the external pump. The master cylinder is what is actually doing the work on the brakes. All the booster does is give the brake pedal more oomph so you don't have to smash the pedal for it to drive the master cylinder.

Booster operates on vacuum. Both RS5 and stock S4 have boosters. They both have vacuum, otherwise Boosters wouldn't work. I didn't say vaccum did not effect the operation of the booster, all I am saying is in theory the manufacturer should stick to certain norms and standards on which many systems are operating. According to you one car has an external pump and the other one doesn't but it doesn't mean that the level of vacuum is any different on which the vacuum system of the engine is operating (including brakes). The vacuum should be regulated to meet a standard value across the board. Can you find and RS5 and data log the vacuum value and do it with your car then compare them, that would be helpful.

I am pretty sure the booster isn't as simple as just sit there to house the MC. I am pretty sure internally its different because most likely it one was engineered for pushing a larger piston which would "fight back" your pedal efforts harder thus perhaps giving you less initial bite and defeat this whole purpose all this research and time. Going this length might as well weigh in all your options and figure out how the RS5 booster differ from B8 because obviously they aren't the same.

[rubicant5x]

Booster operates on vacuum. Both RS5 and stock S4 have boosters. They both have vacuum, otherwise Boosters wouldn't work. I didn't say vaccum did not effect the operation of the booster, all I am saying is in theory the manufacturer should stick to certain norms and standards on which many systems are operating. According to you one car has an external pump and the other one doesn't but it doesn't mean that the level of vacuum is any different on which the vacuum system of the engine is operating (including brakes). The vacuum should be regulated to meet a standard value across the board. Can you find and RS5 and data log the vacuum value and do it with your car then compare them, that would be helpful.

I am pretty sure the booster isn't as simple as just sit there to house the MC. I am pretty sure internally its different because most likely it one was engineered for pushing a larger piston which would "fight back" your pedal efforts harder thus perhaps giving you less initial bite and defeat this whole purpose all this research and time. Going this length might as well weigh in all your options and figure out how the RS5 booster differ from B8 because obviously they aren't the same.

There are no guarantees the RS5 booster that was designed to work with an external pump is going to work with a system that does not have an external vacuum pump. Yes, you are right the booster doesn't just house the MC but it was designed to work with the platform it was installed in, in this case the S4 or RS5. There are too many things to go wrong with trying to retrofit the booster that has an external vacuum source.

I am going to stick with the B8 booster.

[DBFL]

So I ordered the Macan Air scoops and clips. Does anyone have the part numbers for the correct Macan dust shields? Is 95B615311B and 95B615312B correct?

Yep. Those are the parts I used.

[FlyS4]

I think I might have discovered something. Just ordered a used master and booster combo. Its aluminum/unpainted. Came out of 2014 RS5. What is interesting it has the same part number as a B8 booster except of the last letter being "A". I suspect "A" is for the finish. I am not entirely sure but i have a reason to believe B8 booster might be the same as RS5 appart from the black paint. Unless they updated the booster later on for newer models.

Update: this part number is what actually was installed in into the RS5 from the factory then at some point TRW was no longer a supplier so the part number changed. Nothing wrong with an OEM installed unit.