Brake master cylinder size.

[JustHarry]

Changing the brakes on my ms65

Currently it has a good pedal and stops well. Master cyl is 7/8 and one caliper piston is 53mm.  That's about 11.5:1 area ratio. 

What I'm going to is a 4 pot caliper with 43mm pistons. An 11:1 area ratio puts me just over 1" master cylinder. 

Pedal is 4.5:1

Does that sound right to anyone who knows more about this stuff?.

Boosted still 

 

[smokin'joe]

@sr2   i believe you used to fuck with brakes regularly on race/rally cars ?

[mjrstar]

I'm a fan of a big master cylinders, * so long as still boosted (and the booster system isn'tweak),  it gives a nice high pedal, id be looking for a 17/16 m/c. 

I went to this size when swapping my evo to brembo 4 pots, and have since used the same m/c in the mx5.

Also went bigger on my honda, and fine tuned that some more by swapping booster diameter.

 

[sr2]

1 hour ago, smokin'joe said:

@sr2   i believe you used to fuck with brakes regularly on race/rally cars ?

May I start with correcting smokin’joe – I have been regularly fucked over by race/rally car  brakes for a long time!

You’re close with the maths. With a single (i.e. floating) calliper you double the swept area in your calculation. Your standard single piston Toyota calliper has a swept area of 4412 sq/mm.  Your proposed 4 pot has a swept area of 5808 sq/mm. You’re looking at a 31% increase on swept area.

Your 7/8” m/cyl has a swept area of 387 sq/mm and an additional 31% takes you to 507 sq/mm which is close to 1”.

If the 4 pots are of good quality (preferably not Willwood) you may get away with the original 7/8 master cyl – it’s well worth a try. Remember there is minimum fluid displacement with disk brakes; conventionally boosted (i.e. Mastervac) systems normally have a larger master cylinder than you would run with a non-boosted balance bar setup.

Potentially the bigger issue could be the increase in front bias. It’s always harder to predict with a disc/drum setup as the inherent self-energising property of leading shoe throws the numbers out a bit. Once again my advice would be to fit the larger front callipers and do a road test paying attention to pedal travel and bias.

The Golden rule with modifying brakes is to only change one thing at once. Hope this makes sense.

[440bbm]

that should be sticky'd

[JustHarry]

1 hour ago, sr2 said:

May I start with correcting smokin’joe – I have been regularly fucked over by race/rally car  brakes for a long time!

You’re close with the maths. With a single (i.e. floating) calliper you double the swept area in your calculation. Your standard single piston Toyota calliper has a swept area of 4412 sq/mm.  Your proposed 4 pot has a swept area of 5808 sq/mm. You’re looking at a 31% increase on swept area.

Your 7/8” m/cyl has a swept area of 387 sq/mm and an additional 31% takes you to 507 sq/mm which is close to 1”.

If the 4 pots are of good quality (preferably not Willwood) you may get away with the original 7/8 master cyl – it’s well worth a try. Remember there is minimum fluid displacement with disk brakes; conventionally boosted (i.e. Mastervac) systems normally have a larger master cylinder than you would run with a non-boosted balance bar setup.

Potentially the bigger issue could be the increase in front bias. It’s always harder to predict with a disc/drum setup as the inherent self-energising property of leading shoe throws the numbers out a bit. Once again my advice would be to fit the larger front callipers and do a road test paying attention to pedal travel and bias.

The Golden rule with modifying brakes is to only change one thing at once. Hope this makes sense.

I will be adding an adjustable brake bias valve to deal with the rears. 

The calipers are an older jfz 4 post caliper 

Rears are also going from a 9" holden drum to a 10" holden drum assembly 

[JustHarry]

@sr2 I also see you work doubling a sliding caliper vs i was only using 2 pistons on the 4 pot for the math. . The internet suggests both ways

[sr2]

3 hours ago, mjrstar said:

I'm a fan of a big master cylinders, * so long as still boosted (and the booster system isn'tweak),  it gives a nice high pedal, id be looking for a 17/16 m/c. 

I went to this size when swapping my evo to brembo 4 pots, and have since used the same m/c in the mx5.

Also went bigger on my honda, and fine tuned that some more by swapping booster diameter.

 

Which cars were ABS? (I'm presuming all were boosted).

[mjrstar]

8 hours ago, sr2 said:

Which cars were ABS? (I'm presuming all were boosted).

 

yes all boosted

Evo retained abs, honda abs delete,mazda had no ABS, starlet had no abs. Falcon retained abs.

Evo and falcon street cars, mazda,honda, starlet not streeters.

All had/have increased disc diameter and larger piston calipers. And a variation of pad choice.

(with the exception of honda and starlet where the rear went from drums to disc)

My personal opinion is you have to be way more careful with dual master or unboosted setups to ensure you don't end up with crazy pedal effort

Whereas I've found a default 1 inch or slightly bigger provides good pedal effort when boosted.

The secret sauce in braking performance really is the pads. For instance I rolled the dice on some QFM pads for my daily and they are fucking garbage. On that car I'm going to something that's more around the coefficient of friction around 0.5 and will fit some ebc reds shortly. 

 

I went from stock brembo cheese pads on my falcon to a dba street performance (around .40-45 ), it's reasonable but not spectacular given the fronts are on a 355mm rotor. Probably OK for a street car I guess.

 

 

 

[cletus]

3 hours ago, mjrstar said:

 

 

The secret sauce in braking performance really is the pads. For instance I rolled the dice on some QFM pads for my daily and they are fucking garbage. On that car I'm going to something that's more around the coefficient of friction around 0.5 and will fit some ebc reds shortly. 

 

 

 

 

this 100%

 

i dunno how many times ive told people to pass cert brake test, go and buy some good quality pads from race brakes

 

brings car back

fails again

what pads did you get?

oh those fancy pads were 300, i went to repco and they had some high performance pads for 100 so i got those

goes back to race brakes and buys the 300 dollar ones

passes

[mjrstar]

10 hours ago, cletus said:

 

brings car back

fails again

 

 

Can't expect a piece of mahogany between the caliper and disc to provide optimum braking friction.

 

 

[JustHarry]

@mjrstar I remever i have a pbr booster for the crown that a 1" holden master fits so will set up with that and see how it goes 

[sr2]

On 01/09/2025 at 20:50, JustHarry said:

I will be adding an adjustable brake bias valve to deal with the rears. 

The calipers are an older jfz 4 post caliper 

Rears are also going from a 9" holden drum to a 10" holden drum assembly 

 As you’ve gathered, our joint, blindingly accurate mathematical guesstimates have confirmed the 4 pots on the front will considerably increase the front bias.

Be aware that (presumably) the “bias” valve you’re looking at (i.e. one that is plumbed into the rear line) is actually a pressure limiting valve and will only be able to set a maximum limit to the psi the rears receive.

The larger Holden Drums are cool, I’m a fan of Holden drums – PBR nailed it in the day, (I was original trained by PBR in the 80’s in AU) but please at the risk of repeating myself – change only one thing at a time. I have on a number of occasions been asked to diagnose brake issues on both street and competition cars where many components were changed or modified at one time because the builder or more usually the owner thought it would work!

[sr2]

On 02/09/2025 at 06:29, mjrstar said:

 

yes all boosted

Evo retained abs, honda abs delete,mazda had no ABS, starlet had no abs. Falcon retained abs.

Evo and falcon street cars, mazda,honda, starlet not streeters.

All had/have increased disc diameter and larger piston calipers. And a variation of pad choice.

(with the exception of honda and starlet where the rear went from drums to disc)

My personal opinion is you have to be way more careful with dual master or unboosted setups to ensure you don't end up with crazy pedal effort

Whereas I've found a default 1 inch or slightly bigger provides good pedal effort when boosted.

The secret sauce in braking performance really is the pads. For instance I rolled the dice on some QFM pads for my daily and they are fucking garbage. On that car I'm going to something that's more around the coefficient of friction around 0.5 and will fit some ebc reds shortly. 

 

I went from stock brembo cheese pads on my falcon to a dba street performance (around .40-45 ), it's reasonable but not spectacular given the fronts are on a 355mm rotor. Probably OK for a street car I guess.

 

 

 

Hey, great post mate – you make good practical sense.

 We ran an Evo IV for a while and I have to say I was so impressed with the ABS, it coped with 6 Pot AP’s on huge floating disc’s without blinking. The only ABS I’ve been in contact with that was significantly better was on our GT3 but to be honest Porsche shit is so out of the square I never managed to get my simple, bald old head fully around it!

Pads selection has more options today than 30 plus years ago when DS11’s simply didn’t work without heat in them (nothing less than terrifying on a touring stage!). I don’t think the science has changed; softer pads = better bite from cold, less disc wear, but more fade whereas the harder compounds give you the opposite. What has changed however is the modern compounds that bridge the gap – have to say Padgid is my current pad of choice.

[mjrstar]

Interestingly the evo had either a 15/16 or 1 inch (cant remember), which didn't quite hit the spot so that got a 1 1/16 which put it right..

I'm not convinced this situation was all to do with bore size, but it was the only thing I changed. Maybe the original was a tiny bit faulty.

I got the abs unhappy on a couple of occasions, bombing into the hairpin at taupo on smooth semi slicks in standing water ( can't blame the brakes for that though that was tyre friction limited)

 

The other time thinking I was tommi makinen on some hard packed gravel on semi slicks again. ( they were surprisingly decent when up to temp though) once again operator error / youthful exuberance.

Fun times.

[Tiger Tamer]

Good topic. On my minx project I have put Brembo 4 pot calipers from a 406 Peugeot on the front. The rear are 8" drums standard for the avenger diff I fitted. The MC is the Minx mc which has a 3/4 bore which had no power assist. It would be nice to have a twin circuit system and vacuum assist probably a VH44. 

For now I am going with what I have. I did get some softer pads from BNT as I thought they would be better with non assisted brakes, give me a bit more bite. BUT the F@#$%*! things have a taper, so the contact surface is a lot less than the old pads i am replacing. They seem to be negating the advantage of using the softer pad. Might try race brakes to see if they have better.

[smokin'joe]

On 01/09/2025 at 19:43, sr2 said:

If the 4 pots are of good quality (preferably not Willwood) you may get away with the original 7/8 master cyl – it’s well worth a try

the issues with Wilwood being ??
curious as have a set to fit to RWD Levin once it unburied

 

[Bling]

They'll stop it no worries if mine are anything to go by. Probably just better OEM options out there I imagine. I wanted easy bolt on kit, no complaints. Put a new, same size master on at the same time and it works fine too. Wasn't sure if i'd need to mess with it due to going 200mm rotor single pot to 276mm rotor 4 pots, but the pistons were sized to roll factory gear from memory.

[smokin'joe]

10 hours ago, Bling said:

They'll stop it no worries if mine are anything to go by. Probably just better OEM options out there I imagine. I wanted easy bolt on kit, no complaints. Put a new, same size master on at the same time and it works fine too. Wasn't sure if i'd need to mess with it due to going 200mm rotor single pot to 276mm rotor 4 pots, but the pistons were sized to roll factory gear from memory.

can you still run 13" with your kit ?

[Bling]

Nope.