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I am currently upgrading my brakes on my F54 Cooper S using parts from a BMW F45 225i, it is not as sexy as a JCW brake retrofit but rotors size are pretty similar. Only downside is floating 1 piston front caliper instead of the 4 pistons JCW caliper.
I fully upgrade the cars with the following parts, i got a good deal and they are all brand new:
- Front calipers (57mm piston)
- Front rotors (330x24)
- Front pads (OEM BMW)
- Rear calipers (36mm piston)
- Rear rotors (300x20)
- Rear pads (OEM BMW)
- Master cylinder (15/16")
- Brake lines (OEM quality, not from BMW)
In this upgrade, brake booster remains stock, i didn't replace it.
I am facing an issue which is giving a headache. The pedal feels is soft and the pedal travels quite a lot in my opinion.
- Master cylinder has been bench bleed until no air was coming out from the outlet ports.
- Bleeding has been done with a pressure bleeder (~ 30psi)
- DSC module bleeding has been done with ISTA
I repeated the operation multiple times, no air bubble is coming out from any caliper. I even tried to gently hit the dsc module, brake calipers with a rubber hammer to try to dislodge any stuck air bubble but nothing more came out.
Engine on, at rest, the brake pedal sits at 6.1" from the floor. I have around 0.6" of loose travel until the brake lights switch on and the automatic gear lever is unlocked. From there, i'd say the pedal is really soft for the next 1.5" of travel.
From rest, the brake pedal travels around 2.1" until i can i feel a resistance. It doesn't make me confident about the brakes. But even with this inconvenience, the car still stops fine, but i didn't tried any emergency braking since i didn't finish to bed in the pads. Engine off, the brake pedal travels slightly less than engine on but the travel occurs with resistance until i am not able to push anymore.
Pumping the brakes repeatedly in the short window of time does not increase resistance nor reducing the brake pedal travel, which makes me think i don't have air in the system.
One thing i also noticed is that when the pressure bleeder is on, set to ~ 30psi and all bleeders are closed, if i push the pedal, it only travels the free play distance, which is around 0.6" and then it is rock solid.
The thing is i don't know if this is the normal brake pedal behaviour on 3rd gen Mini ? Do I still have air bubble somewhere ? Did i miss something ? Could the brake booster can be the cause as i didn't replaced it ?
On RealOEM, the brake booster reference is different between the S and JCW, could the JCW one fix my problem ? I didn't bother about the brake booster until as his job is only to assist the brake pressure, i don't know in what way it could affect the brake pedal travel.
Looking around, looks like the stock F54S front calipers have 54mm pistons. You noted your upgraded calipers have 57mm pistons. That is an 11.4% increase in area of the piston, so the master cylinder has to push more fluid for the same amount of piston displacement...which will increase your pedal travel when keeping the same master cylinder. My guess is that the BMW F45 has a larger master cylinder piston or potentially a different mechanical ratio on the brake pedal pivot. Only way to go back to a 'stock feeling' pedal would be to get a properly matched master cylinder in there, IMHO. Do your research and see what you can find on the master cylinder and brake pedal ratios and plan accordingly.
I have done a complete swap from the F45 225i, that means from the F54s, front piston diameter increased from 54mm to 57mm, and rear piston diameter 34mm to 36mm.
In order to prevent longer pedal travel due to an increase piston area, I also replaced the master cylinder with the corresponding one, that means an upgrade in size from 7/8" to 15/16".
Pads are now bedded in, the feeling is a bit better and brake feeling is fine under light to medium braking but the feeling under hard braking still gives me long pedal travel and i am not very confident. I haven't tried an emergency stop to see if i can trigger the ABS.
Next step will be to replace the brake lines with new ones. Currents are new but poor quality IMO, i am going to replace them with a good brand like ATE, TRW.. And see if any improvement occurs.
Unfortunately, SS braided lines is not an option as they fail inspection and not legal on open roads.
Based on your swap info, you should have 'good' pedal travel/feel, but still don't. I wonder if the F45 225i has a different brake pedal arrangement, where the pivot points are different and the mechanical pedal travel is different from your F54?? If not, I would have to believe you still have some air hiding in the system. Bench bleeding a master cylinder can work, but doesn't guarantee you don't end up some air still hiding. I have read various posts over the years of folks having issues bench bleeding master cylinders. Luckily this is one area on vehicles I haven't had to work with myself, so I can't offer any real advice. I have always vacuum bleed my brake systems, including my F54 and have never had air issues, FWIW. Hope you resolve your issue.
That's a good point about the pedal arrangement but what makes me doubt about it is that the F54 JCW (or S with sports brake retrofit) has the same pedal assembly as the S. The F54 JCW also shares the same master cylinder as the F45 225i. Rear calipers are same exception the color and front ones have a slightly more total piston area than the JCW brembo 4 pistons calipers, but really slighty more, we are talking about a few mm2 more. It shouldn't affect the feeling that much.
So like you said, I should have a good pedal travel/feel.
I also did bench bleed the new master cylinder before installing it, but I am starting to think same as you said, maybe some air is still hiding in the system despite pressure bleeding and DSC bleeding with ISTA.
Next step will be a replacement of the poor quality rubber lines, I went cheap on them but I should have not. I have one spare and the end fitting inner diameter is smaller than the OE specifications. I cutted one and reinforcement looks poor, so maybe bad feeling comes from lines expansion.
I bought good quality ones from a well known OE brand and will re bleed the system and I hope some improvements will occur.
I am currently upgrading my brakes on my F54 Cooper S using parts from a BMW F45 225i, it is not as sexy as a JCW brake retrofit but rotors size are pretty similar. Only downside is floating 1 piston front caliper instead of the 4 pistons JCW caliper.
I fully upgrade the cars with the following parts, i got a good deal and they are all brand new:
- Front calipers (57mm piston)
- Front rotors (330x24)
- Front pads (OEM BMW)
- Rear calipers (36mm piston)
- Rear rotors (300x20)
- Rear pads (OEM BMW)
- Master cylinder (15/16")
- Brake lines (OEM quality, not from BMW)
In this upgrade, brake booster remains stock, i didn't replace it.
I am facing an issue which is giving a headache. The pedal feels is soft and the pedal travels quite a lot in my opinion.
- Master cylinder has been bench bleed until no air was coming out from the outlet ports.
- Bleeding has been done with a pressure bleeder (~ 30psi)
- DSC module bleeding has been done with ISTA
I repeated the operation multiple times, no air bubble is coming out from any caliper. I even tried to gently hit the dsc module, brake calipers with a rubber hammer to try to dislodge any stuck air bubble but nothing more came out.
Engine on, at rest, the brake pedal sits at 6.1" from the floor. I have around 0.6" of loose travel until the brake lights switch on and the automatic gear lever is unlocked. From there, i'd say the pedal is really soft for the next 1.5" of travel.
From rest, the brake pedal travels around 2.1" until i can i feel a resistance. It doesn't make me confident about the brakes. But even with this inconvenience, the car still stops fine, but i didn't tried any emergency braking since i didn't finish to bed in the pads. Engine off, the brake pedal travels slightly less than engine on but the travel occurs with resistance until i am not able to push anymore.
Pumping the brakes repeatedly in the short window of time does not increase resistance nor reducing the brake pedal travel, which makes me think i don't have air in the system.
One thing i also noticed is that when the pressure bleeder is on, set to ~ 30psi and all bleeders are closed, if i push the pedal, it only travels the free play distance, which is around 0.6" and then it is rock solid.
The thing is i don't know if this is the normal brake pedal behaviour on 3rd gen Mini ? Do I still have air bubble somewhere ? Did i miss something ? Could the brake booster can be the cause as i didn't replaced it ?
On RealOEM, the brake booster reference is different between the S and JCW, could the JCW one fix my problem ? I didn't bother about the brake booster until as his job is only to assist the brake pressure, i don't know in what way it could affect the brake pedal travel.
Any help will be much appreciated ! Thanks !
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Based on what you’ve described, it does sound like the brake booster could be playing a role in the excessive pedal travel and softness. While a brake booster mainly assists with pedal effort, differences in booster design (like the one in the JCW) could affect pedal feel, particularly if the booster in your Cooper S isn’t optimized for the increased piston sizes in your retrofit.
That's a good point about the pedal arrangement but what makes me doubt about it is that the F54 JCW (or S with sports brake retrofit) has the same pedal assembly as the S. The F54 JCW also shares the same master cylinder as the F45 225i. Rear calipers are same exception the color and front ones have a slightly more total piston area than the JCW brembo 4 pistons calipers, but really slighty more, we are talking about a few mm2 more. It shouldn't affect the feeling that much.
So like you said, I should have a good pedal travel/feel.
I also did bench bleed the new master cylinder before installing it, but I am starting to think same as you said, maybe some air is still hiding in the system despite pressure bleeding and DSC bleeding with ISTA.
Next step will be a replacement of the poor quality rubber lines, I went cheap on them but I should have not. I have one spare and the end fitting inner diameter is smaller than the OE specifications. I cutted one and reinforcement looks poor, so maybe bad feeling comes from lines expansion.
I bought good quality ones from a well known OE brand and will re bleed the system and I hope some improvements will occur.
With the pedal/master cylinder arrangement/relationship, then you should be OK there. Hopefully that 'slightly' larger piston area isn't really playing that much of a role, but something to keep in mind as you tackle other points. Start with the new hoses and see where it gets you, since you will also be bleeding the system again as well. Hopefully you won't have to resort to pulling the master cylinder again to re-bench bleed it. Good luck and let us know what you find.
