Calling for help - 300 to 325mm brake upgrade

C

ChrisGUK

Member
Hi all. I’ve done the classic 3.0i to 3.0si front brake upgrade. It all went reasonably smoothly from a physical perspective, only a few stubborn bolts I had to get some friends to help with, and then some finessing of the dust plate as it was rubbing on tight corners.

This problem is. I’ve bled the whole system twice but I still have a loooong and moderately soft brake pedal. I’m going to do another bleed, super thoroughly and drain through a load of fluid to ensure it’s Really Done this time.

BUT. I’ve since noticed that the new (325mm) calipers take a 57mm piston vs a 54mm on the stock (300mm). Simple physics tells me this will need more fluid to generate the pressure to displace it. I didn’t check before (because I’m a fool and I just trusted ‘the Internet’) but now I have gone and double-checked and the master cylinder for the 3.0i and the 3.0si is definitely the same part (according to realOEM). I haven’t done the sums but I can’t imagine 3mm diameter is going to amount to much additional fluid/pressure - and BMW obviously felt the same.

So I’m kind of stuck wondering, those of you who have done the brake upgrade, do you just accept that you have a slightly longer brake pedal?
Or have I just done a bad job with bleeding the system twice in a row?
Or is there something else I could be missing?

Please help!

(It does look cooler, which was at least 50% of the motivation).

IMG_6938.jpegIMG_6940.jpeg
 
Did you let the pipework empty to the point where you may have air in the ABS pump? Once air gets in then it's the very devil to get it out again. Either a power bleeder or there's a semi-automated one in INPA (or is it an ISTA/DIS thing? I forget.)
 
I’ve done this change several times and never had an issue

Every time I’ve had long pedal it’s been air

It’s worth using inpa to properly bleed the system in case you have air in the unit
 
smorris_12 said:
Did you let the pipework empty to the point where you may have air in the ABS pump?
Click to expand...

Pretty sure I didn’t, I kept the top reservoir full with top-ups the whole time.

bigwinn said:
I’ve done this change several times and never had an issue
Every time I’ve had long pedal it’s been air
Click to expand...

That’s good news that at least a fix is achievable! I’m assuming the air won’t/shouldn’t migrate upwards? I’ll see if I can find a suitable helper or maybe just pay someone to make sure it’s done right.

The other thing I have to double check is my disc thickness - new is supposed to be 25mm vs 22mm. I’m sure it’s the right diameter but I didn’t actually measure the thickness! But it sounds like air is in need of more work.

Thanks to you both.
 
Good luck bud you’ll sort it

Just a thought- and I’m saying as I’ve done it

Your calipers are on the right side yes?

If not you won’t get the air out
 
bigwinn said:
Your calipers are on the right side yes?
Click to expand...

Thanks!

Regarding side, I certainly hope so! I’m not with the car at the moment, but I guess it would have to be the wrong way up (lines down) to be able to bolt to the wrong side of the car?
 
Well I managed it on the rears…

I’m not saying it’s probable but worth a look
 
Bigger diameter piston won’t affect the pedal travel. Any soft/long pedal will be air in the system, assuming there are no leaks. Make sure your brake nipples are closed and (if you are doing a thorough investigation) your caliper seals are intact.
 
Zedebee said:
Bigger diameter piston won’t affect the pedal travel. Any soft/long pedal will be air in the system, assuming there are no leaks. Make sure your brake nipples are closed and (if you are doing a thorough investigation) your caliper seals are intact.
Click to expand...
To be pedantic, if you are now moving a bigger piston through the same distance by applying force from the original size input piston then you need to displace more fluid to do so. So technically the pedal will need to move a little bit further, but it is a minimal amount.
The issue in this case is still most likely to be air in the abs unit. Until that is bled you are clutching at straws.
 
Thanks chaps. I’m just going to take it to a shop and get them to do it for me. I fiddled about enough.

Actually, the first thing I’m going to do is make sure that my brake disc is the right thickness as I didn’t check that before installation.
 
ChrisGUK said:
Thanks chaps. I’m just going to take it to a shop and get them to do it for me. I fiddled about enough.

Actually, the first thing I’m going to do is make sure that my brake disc is the right thickness as I didn’t check that before installation.
Click to expand...
That won’t make a difference to pedal feel once the pad is pushed up to it. It’s only rotational force of the disc that moves the pads back so they don’t go all the way back in, just enough to allow the disc to rotate.
 
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