My deluxe has had disc brakes with no power booster for the past 30 years or so. Stops well but has always had a lower than normal pedal when pressed. It still has the standard 3/4 inch rear wheel cylinders. If I go to 5/8th wheel cylinders, wiil I get a better pedal. Also will these fit the backing plates or is a modification required.

Thanks in advance.

I don't think you need to go that drastic to get firmer pedal feel. Back brakes cylinder size don't have much of an effect to higher pedal, lots of other stuff does. Suggest you look at those first. I recently overhauled 90% of the brakes on my car, every little bit counts. This is what I did and I have excellent pedal feel:

- New master cylinder (You can rekit yours, but my tin tank broke it's lid screw part)
- Replace the rear adjuster screws and wedges <--- this made the biggest single difference other than the MC change
- Replaced front and rear brake hoses <--- this is important, they do internally bulge apparently and give you bad pedal feel
- Replace front caliper seals, better still just rebuild the front calipers
- Replace clevis pin and generally get rid of all play in the pedal
- Change to Green Stuff pads, I had black stuffs and the Greens are superior bite and fade much less
- Bleed the system properly... There are many ways to do this and the single best way I find is still to have someone step on the pedal method
- Last but not least, check and replace the rear slaves, check the drum wear and shoe wear, adjust them properly with the new wedges and adjuster screws

If everything is in tip top shape, you should have excellent brakes. Going to bigger rear wheel cylinders is sort of trying to lipstick on a pig as the old saying goes.

If after all this you are still dissatisfied with pedal feel then it's time to get a booster!

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Lillee - 1969 Morris Mini K

I had 3/4 rear cylinders on Barney with S front brakes and no booster. After a major drama on a freeway in the wet at 110kmh (rear wheels locked then car weaved during heavy braking)) I fitted 5/8" rears.
That's the reason the Cooper S had 5/8" rear cylinders, SAFETY. Not to get a firm pedal.

The 5/8 cylinders have the dowel pin in the other end compared to 3/4 ones. You either need to re-drill the backing plate or do what I did- drill another 5/32" hole in the cylinder and move the pin. However if it is an S, the backplate holes are correct already.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

+1 - spun in the wet and ended up facing the traffic

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Videos - Turbocharged A-Series on Dyno & at Wakefield
Mafs waz not my strong subgeckt at skkol

+1 - may have contributed to a written off MK2 S and nearly my life...

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did I tell you that I won a trophy?

Not sure I understand what is going on here. The cooper S slave is a 5/8 bore and the standard one is a 3/4 bore which is equivalent to a 6/8 bore. The Master Cyl emits a said volume of fluid. In a larger bore (the 3/4 one) it requires more fluid to move the pistons of the slave outwards due to the increased volume of the slave cylinder. Therefore you need to depress the pedal further in order to actuate the slave to the same distance as that of the smaller diameter 5/8 slaves. If this is the case with fluid dynamics then yes switching to a smaller diameter slave will require less pedal travel to activate the rear shoes. You would be more likely to lock a rear wheel with a small cylinder than a larger one. You can apply more pressure and better braking affect on the rear wheels with the 5/8 cylinder.

There is no doubt that changing to the 5/8 cylinder WILL reduce pedal travel.

Why you guys are locking rear wheels with 3/4 slaves is anybodies guess, but I would say it is more likely because of a lack of maintenance and your rear drums are out of round. Somebody please correct me if I am wrong.
Greg

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1970 Cooper S ex-Bathurst & ATCC
1964 Austin Cooper S ex-Group C race car
1967 Morris Cooper S ex-Group B
1962 Mini Speed sports sedan
1968-71 ex-Peter Manton Shell car

Bigger cylinder has more surface area, force at shoes = pressure x area. So brakes lock with less pedal pressure.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Hopefully people will read this and try to understand before arguing with you!!!

After careful consideration I have to admit that my initial thoughts on this where incorrect

The force (in pounds) exerted by the wheel cylinder on the brake shoes is line pressure (pounds per square inch) times the area of the piston in the cylinder (square inches). More square inches (meaning a larger piston), more pounds of force.

Dave

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Kevin,
Are you kidding. Would you like me to do the maths for you?
Sorry that was harsh, p1v1/t1 equals p2v2/t2 given temp is constant the pressure is inversely proportional to the vol.

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1970 Cooper S ex-Bathurst & ATCC
1964 Austin Cooper S ex-Group C race car
1967 Morris Cooper S ex-Group B
1962 Mini Speed sports sedan
1968-71 ex-Peter Manton Shell car

The S brakes were fitted with a lower pressure limit valve (softer spring) & smaller rear cyls simply to help stop the rear brakes locking up (IE: to make them work "less" effectively)

this "combination" is perfect when discs are fitted to the front,
the factory did their job right

"IF" you still have large rear cyls & the std "mini" limit valve spring, you "WILL" have locking rear brakes with discs fitted on the front, especially dangerous in the wet

simple as that !!!

If you have a low peddle, then you have an adjustment issue of some sort,,, shoe or pad travel is exsessive for some reason

if you have a "spongey" peddle, then you have air in the system (generally) or a bend brake pad or swollen/guey hoses etc

if you have a low & spongey peddle, then you have a combination of all of the above

again---> simple as that

there are some people who have fitted the larger master cyl (.75)
& this will give a much higher, firmer peddle, when everything else is perfect
but then more peddle pressure is needed.

edit--> but to answer your question, yes fitting the smaller rear cyls will give a slight increase in active peddle height, provided everything else is in perfect working order, but it will be barely noticeable.

one thing people often forget about is the amount of movement the rear limit valve piston has, this does creat slight bit of peddle travel depressing the spring/piston inside it, quite often heard & felt.

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

Basically I have done all of the above except for the booster. Been good for 30 years, just want a better pedal. Everything has been replaced, even 3/4 rear cylinders. Never had a problem with locking up, but may replace these with 5/8. The limiting valve is original, appears to be ok, should I replace this anyway. Had pressure bled the system, and the good old pump up, hold, foot to the floor, bleed method, so definitely no air in the system.

Thanks everyone for replying so quickly. Very much appreciated for your expert advise.

Sorry Greg but Doc's right (Pascal's Law for fluids). You've used Boyle's Law for gases...

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Cheers,
Rod.
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'63 Morris 850 (Old Secondhand CAR)

Ah, the old PV=nRT rule. R=8.31, n= number of moles, T=temp, P and V pressure and volume.
I remember from form 6 Chemistry 1979, yikes! I forgot how bloody old I'm getting and I thought my memory was worse than that!

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67 Deluxe
Dry suspension
1293cc, 11:1 comp, RE 282 camshaft, 45 Weber, extractors etc
4 synchro close ratio gbox
Cooper S brakes, no booster
Stealth box, as Doc says "goes harder and idles!"
RH tank cos I needed it crossing the Nullarbor etc..

Aye, this is an incompressible fluid exerting a force on an area so the Boyle's law (adiabatic thermodynamic applications) does not apply. Temperature has nothing to do with force applied as you say, but neither does the volume! Simple pressure applied to area problem.

The volume only determines the amount of pedal movement vs piston movement, not the force applied.

cheers
michael

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the world is a book, and those who do not travel read only a page
66 Mini Minor sponsored by http://www.lifeonthehedge.com.au/ The Dog Harness Specialists
It was a pleasure ausmini. I'll miss all you misfits and reprobates

Love this thread.

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.