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Which Rear Brake Cylinders Should I Be Using?


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#1 Deathrow

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Posted 11 February 2019 - 03:23 PM

This is probably another post where I'm spending a significant amount of time thinking about something that others may not even think twice about but there we are.

 

After doing some reading I've found that there seems to be at least three different sized cylinders available for the rear brakes and I'm just wondering if I need to make a change to a different size being that I've changed the front callipers to Cooper 'S' units.

 

I've made a spreadsheet which calculates the effort front and rear for a given scenario.

 

brake-effort.png

 

M/C - Size of front and rear bores of master cylinder

Front - Size of calliper bore and the piston surface area ratio to the relevant master cylinder bore

Rear - Size of brake cylinder bore and the piston surface area ratio to the relevant cylinder bore

 

The ratio is worked out as 'Calliper piston face surface area / Master cylinder piston face surface area' so for the standard fronts every 1 pound on the master cylinder will result in 16.33 pounds at the disc.

 

This doesn't take in to account the brake limiter valve, however I'm unsure what effect this would have on the braking balance.

 

Should I be thinking about swapping from the 0.750" cylinders to the 0.625" cylinders to ensure the rear isn't over braked?

 

I'm going to have to fit one or the other anyway as I'm relatively certain the two I have on there currently are mismatched. I'm just not sure whether to stick with the 0.750" standard or downsize to 0.625" as fitted to the Cooper 'S'?



#2 nicklouse

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Posted 11 February 2019 - 03:36 PM

ok scroll down http://www.minispare...rt.aspx?1~9~105

gives the sizes that the factor used with what.

 

edit.

 

the rear regulator stops the pressure to the rear when a set figure is reached so then all pressure only goes to the front. so your chart is somewhat wrong. (maybe the wrong word but I think you know what I mean)

 

what MC are you going to be using?


Edited by nicklouse, 11 February 2019 - 03:44 PM.


#3 Ethel

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Posted 11 February 2019 - 03:57 PM

I suspect the S rear cylinders were downsized so the pedal was less likely to hit the floor. They'd also counter some of the effect of the servo, though the pressure limiting valve out to take care of that. As a sports model the bias will be more towards heavy braking performance and less towards having a full back seat and boot.

 

Are you sure on the S caliper pistons diameters, I thought they were closer to 8.4's?

 

This diagram shows the limiting valve is almost entirely operated on the spring preload. The pressure on the rear cylinders when it closes will be the same and the force they apply to the shoes proportional to their relative piston areas. However, not necessarily proportional to the braking effect via a self servoing leading edge shoe.

clip_image0143.jpg?imgmax=800

 

 

Nick, I think it'll be a yellow band master - stepped bore.



#4 Deathrow

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Posted 11 February 2019 - 04:26 PM

ok scroll down http://www.minispare...rt.aspx?1~9~105
gives the sizes that the factor used with what.
 
edit.
 
the rear regulator stops the pressure to the rear when a set figure is reached so then all pressure only goes to the front. so your chart is somewhat wrong. (maybe the wrong word but I think you know what I mean)
 
what MC are you going to be using?

 
 I know what you mean Nick. I wasn't sure of the exact operation of FAM7821 (this one) so omitted it. If FAM7821 limits the rear pressure at a given point, can I assume the rear will behave the same regardless of the change of front callipers? The rear will brake the same but the fronts will require more pedal pressure to achieve the same pressure as standard because of the smaller callipers?

I'm using the yellow tag master cylinder that was standard to my car GMC227 (this one).
 

I suspect the S rear cylinders were downsized so the pedal was less likely to hit the floor. They'd also counter some of the effect of the servo, though the pressure limiting valve out to take care of that. As a sports model the bias will be more towards heavy braking performance and less towards having a full back seat and boot.
 
Are you sure on the S caliper pistons diameters, I thought they were closer to 8.4's?
 
This diagram shows the limiting valve is almost entirely operated on the spring preload. The pressure on the rear cylinders when it closes will be the same and the force they apply to the shoes proportional to their relative piston areas. However, not necessarily proportional to the braking effect via a self servoing leading edge shoe.
clip_image0143.jpg?imgmax=800
 
 
Nick, I think it'll be a yellow band master - stepped bore.


No servo for me Ethel, though I understand I'll still achieve the same braking effort without the servo but with less pedal travel?

As standard my car had no servo and it had a very firm pedal that I'm keen to retain. The only changes I've made so far are the Cooper 'S' callipers so I can fit 10" wheels.

I guess the rear of the system hasn't changed so they should behave the same as they did previously? I'm just keen to avoid finding the car wants to swap ends because the rears are locking up under braking.


Edited by Deathrow, 12 February 2019 - 12:16 PM.
Updated to include Ethel's edited post.


#5 Hedgey

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Posted 11 February 2019 - 04:28 PM

https://www.calverst...ias-adjustment/

 

Some useful information from Kieth Calver here

 

Elliott



#6 nicklouse

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Posted 11 February 2019 - 06:51 PM

 

ok scroll down http://www.minispare...rt.aspx?1~9~105
gives the sizes that the factor used with what.
 
edit.
 
the rear regulator stops the pressure to the rear when a set figure is reached so then all pressure only goes to the front. so your chart is somewhat wrong. (maybe the wrong word but I think you know what I mean)
 
what MC are you going to be using?

 
 I know what you mean Nick. I wasn't sure of the exact operation of FAM7821 (this one) so omitted it. If FAM7821 limits the rear pressure at a given point, can I assume the rear will behave the same regardless of the change of front callipers? The rear will brake the same but the fronts will require more pedal pressure to achieve the same pressure as standard because of the smaller callipers?

I'm using the yellow tag master cylinder that was standard to my car GMC227 (this one).
 

I suspect the S rear cylinders were downsized so the pedal was less likely to hit the floor. They'd also counter some of the effect of the servo, though the pressure limiting valve out to take care of that.


No servo for me Ethel, though I understand I'll still achieve the same braking effort without the servo but with less pedal travel?

As standard my car had no servo and it had a very firm pedal that I'm keen to retain. The only changes I've made so far are the Cooper 'S' callipers so I can fit 10" wheels.

I guess the rear of the system hasn't changed so they should behave the same as they did previously? I'm just keen to avoid finding the car wants to swap ends because the rears are locking up under braking.

 

right got where we are. funny that as i asked something similar 

http://www.theminifo...lain-something/



#7 Moke Spider

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Posted 11 February 2019 - 07:19 PM

A very simple 'chart' I have is;-

 

Front Brakes                                        Rear Wheel Cylinder

 

Drums (Single Leading Shoe)          Convert to Twin Leading Shoe !

Drums (Twin Leading Shoe)             0.750"

Disc 7.0"                                           0.625"

Disc 7.5"                                           0.625"

Disc 8.4"                                           0.6875"

 

These are 'safe' numbers as it's unlikely, when used with a form of rear limiter valve. that the rears will lock. You can play around with them and also the valving and type of valve to fine tune it, as other factors like weight distribution, the height of the weight (increasing / decreasing pitch) and tyres come in to play.

 

Unless you have a 'lot' of weight in the rear, there's little point in increasing the braking capacity down that end.



#8 absx2

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Posted 11 February 2019 - 08:27 PM

Drums (Single Leading Shoe)          Convert to Twin Leading Shoe !      Sooo true  :D



#9 Deathrow

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Posted 12 February 2019 - 01:15 PM

Thanks for your input everyone.

Ethel, I'm using MiniSpares replacement calliper pistons (8.4" piston, 7.5" piston) for calliper bore sizes as I couldn't find calliper specs anywhere.

If I'm understanding the operation of FAM7821 correctly now then it essentially does nothing until x line pressure and then it directs all additional pressure to the front brakes. So we could say that my table in the first post is applicable while line pressure is less than x.

In that case I think the toss up is between:

  • 'S' callipers + 0.625" cylinders - shifts balance 0.91% towards the front, reduces overall brake effort by 24.21% (compared to 8.4" + 0.750" cylinders).
  • 'S' callipers + 0.6875" cylinders - shifts balance 0.94% towards the rear, reduces overall brake effort by 22.62% (compared to 8.4" + 0.750" cylinders).

Based on that and recommendations it looks like 0.625" is a good start point, moving balance away from the rear (so not increasing chance of lockups) and keeping the balance front to rear as close to standard as possible.

I'll get some ordered :proud:.

Thanks everyone.


Edited by Deathrow, 12 February 2019 - 01:28 PM.
Corrected numbers.


#10 nicklouse

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Posted 12 February 2019 - 01:22 PM

I would not bother with the FAM7821 at all just fit a rear mounted regulator and mod t to be adjustable for the shut of pressure you want. or buy it ready done.

http://www.minispare...A1774.aspx|Back to



#11 nicklouse

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Posted 12 February 2019 - 01:25 PM

also interesting reading.

https://kentautodeve...ant=33659807879

 

3 Levels of kit can be supplied - 

  • Plain Box - you will need to supply your own master cylinders
  • With Master Cylinders - comes with integral master cylinders 5/8" for the front and 3/4"  rear. Suits most Minis.
  • Complete With In-Car Adjuster - as above with internally mounted cable able to fine tune braking on-the-move. 
  • With AP Racing Master Cylinders - Comes with AP Master Cylinders to suit the lighter weight Minis

The master cylinder bore sizes supplied will suit cars with a standard brake system and standard weight distributions. Please ask for our advice for any non-standard variations in foundation braking system and weight distribution.

 



#12 Moke Spider

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Posted 12 February 2019 - 06:08 PM

I would not bother with the FAM7821 at all just fit a rear mounted regulator and mod t to be adjustable for the shut of pressure you want. or buy it ready done.

http://www.minispare...A1774.aspx|Back to

 

I agree with Nick here.

 

The FAM7821 works as you've described, however I am a firm believer in the KISS (Keep It Simple Stupid) system too, especially with brakes. I don't like to have more there than what's needed.

 

While the FAM7821 does balance off the shut off pressure by measuring Front Pressure as well as Rear Pressures, the Tandem Master Cylinder already does this. These valves only duplicate that.



#13 Deathrow

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Posted 22 February 2019 - 12:43 PM

Thanks for the responses guys, I've not had a chance to reply until now.

It does seem like FAM7821 is recognised as being a bit of a pain in the backside. I didn't want to deviate too far away from the standard setup in one great leap and I'd already purchased a replacement FAM7821 and fitted it. Once the car is on the road, if it gives me grief I'll look to swap it out.

I fitted the new 0.6875" (11/16") rear cylinders last weekend. I had to drill the brake backplates to accept the pin being on the other side.

 

After using the Gunson eezibleed kit (what a dream in comparison to a 'vizibleed' kit with the one way valve) to bleed all 4 corners I've got a much improved pedal now. It still feels significantly softer than I remember it did previously. I'm not sure if that's the changes I've made to the brake system, if it is FAM7821 already causing me bleed issues or if I've still got air trapped somewhere.

Unless anyone has any suggestions, I'll leave it as it is now until I can test the brakes with the car moving. I can always bleed them through again as I've got plenty of fluid.



#14 Ethel

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Posted 22 February 2019 - 01:39 PM

I'd say the point of the FAM valve is it only  limits the rear brakes if the fronts have pressure. It catches people out by operating when they're trying to bleed the system, one reason why I prefer pulling the fluid through with a syringe.

 

The 8.4" setup was designed to replace 7.5's without needing a servo. If you want to make comparisons I guess it could be done using drum braked cars as an intermediary as they were used with both sets of hydraulics, pedals 'n valves. They used 3/4" with both, that suggest to me you could do the same as late model servo'd 8.4's will bite harder than drums and 7.5's and also did it using 3/4" wheel cylinders via the FAM thingy.

 

Going smaller will get you a bit sharper pedal, but will only benefit stopping if you're already in the habit of locking up the rear end.






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