Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
1/22/23 9:46 a.m.

Working on a Mini Cooper S.  It has a master cylinder that is really stubby, looks like this:

It had the usual split diagonal brake system with 6mm lines to the ABS unit and 4.75mm lines out to the wheels.  Brake piston diameters were 48mm front and 34mm rear.

I say "were" because the car now is getting calipers with "54mm" (2 1/8") pistons in the front and "38mm" (1 1/2") in the rear.  That is one thing.  The other thing is that, to make life and plumbing a lot easier for the hydraulic handbrake, the system will be changed to a front/rear split.  Front brakes getting 1/4" line to a tee and then 3/16" out to the calipers, rears will be 3/16" all the way back since I know from experience that works just fine.


What I don't know from experience is how well one of these little stubby master cylinders works stroke-wise once one circuit is no longer feeding a 48mm and a 34mm caliper and is instead feeding two 54mm calipers.  I can't see it running out of travel, but I also have never used an "even volume" master on a front/rear split before, and if I am going to change anything now is the time.

I suppose it could also just be a case of way overthinking things...


jfryjfry SuperDork
1/22/23 10:22 a.m.

I'd imagine (and I'm interested to see Angry's input) that the current master has equal output on each circuit which would/should now give you more rear bias, requiring a proportioning valve on that circuit.  Unless there is  a stock proportioning valve still in the system that I'd imagine could work for  you. 

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
1/22/23 10:28 a.m.

No proportioning valve (initially) since Subaru, the brake donor, tend to be excessively forward biased.  I also just remove them on my Mazdas and the brakes work better because Mazda was excessively forward biased.


More trivia.  The Mini master cylinder is .875" bore, the Subaru had a 1" bore (!), which was the smaller of the two options (!!).  This is really curious because Subaru have generally always had really soft, mushy brake feel, I didn't realize they used master cylinders that large.  They must make it up with brake assist.  I have the brake pedal from the Subaru in the parts pile, now I am really, really curious what the pedal ratios are.

rslifkin UberDork
1/22/23 10:31 a.m.

In reply to Pete. (l33t FS) :

The mush in the Subaru pedal feel could be due to other things like flex in the caliper assemblies, brake soft lines, etc. 

bentwrench UltraDork
1/22/23 10:49 a.m.

Flex in the booster itself...

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
1/22/23 10:55 a.m.

...BMW E90s are front/rear split and they use the same master cylinder.  With different bore sizes, in case I need to play with that.


Moving on...

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) GRM+ Memberand MegaDork
1/22/23 11:51 a.m.

Simplified calculation to compare volumes:

Old diagonal circuit 1: ((48)^2 )+((34)^2) = 3460

Old diagonal circuit 2 = same

New front circuit = ((54)^2)+((54)^2) = 5832

New = (5832/3460) = 1.68x old

My experience: we generally select master cylinders with about 2x the circuit volumes, to allow for variations in friction vs temp (ie fade), humidity, etc.

My opinion: your proposed new arrangement only has 15% volume reserve for output variation on the front circuit. That assumes the Mini MC followed the 2x rule. I would not run that on the street. I also wouldn't run that on a track car. It might be OK for autocross.

Options: find a car that was front/rear split with 54mm front calipers, and check the bore and stroke of that MC, and use that info to guide your choice.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
1/22/23 12:28 p.m.

I think I see what you are getting at, thanks.  Very useful info.

The E46 (not E90, my bad) 325i uses this master cylinder with a .937 bore (15/16" just like God and Girling intended) and 54mm front pistons, 40mm rear.


Since there is a bolt in way forward if this does not work, I'll go with what I got for now and if the pedal is crap, I'll worry about it then.  Application is not for street, track, or autocross, and I fully expect the power brake booster to be depleted on a regular basis so having some additional hydraulic leverage may not be unwelcome.


This is one of the few times that a BOLT IN solution actually exists for this car smiley

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
1/22/23 6:44 p.m.

Just because I said I would.

The Mini brake pedal has a truly atrocious 3.5:1 pedal ratio (a tad under 3" to pushrod, roughly 10.5" to middle of pedal). 

The Subaru brake pedal, on the other hand, has a merely icky 3.9ish:1 pedal ratio.  2.5" to a little under 10".

I drilled out my '81's pedal for a 7:1 ratio in anticipation of converting to manual brakes... stock is 4.5:1 and it sucks when the booster is empty.

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