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Best master cylinder for 4 pot brakes? 0.70? 0.75?
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Author:  MattE [ Mon Oct 05, 2009 5:20 pm ]
Post subject:  Best master cylinder for 4 pot brakes? 0.70? 0.75?
Gday all.

I haven't yet got my car on the road with its new Metro (4 pot) calipers. Nor have I been able to get a very good pedal with the bleeding I've done the old fashioned way.

Unfortunately the 0.75" master cylinder has decided to start leaking, so I'm wondering if this is an opportunity for me to change over to a 0.75" jobbie. Surely this is the better size to push the additional fluid required by the 4 pot calipers? The wheel cylinders are the smaller, Cooper S size (5/8"?)

If not the 0.75", then what size would do the best job?

The pedal I had experienced was far from confidence inspiring, although the local brake shop did highly recommend a pressure bleed. Something about not wearing the master cylinder with a squillion applications, and that the bleed nipple is at a low point at the rear. (Can't say I've ever had an bleeding trouble in the past though).

Thanks a lot for any advice.
Matt
Author:  meeni [ Mon Oct 05, 2009 5:27 pm ]
Post subject: 
pressure bleeding wont really help, the 4 pots have shocking pedals with standard m'c's im not sure of what size is best....
Author:  michaelb [ Mon Oct 05, 2009 8:24 pm ]
Post subject: 
Hi Matt,

Check out a few of the UK websites as they also have a bit of info on fitting different brakes.
http://www.turbominis.co.uk/forums/inde ... tid=208560
Author:  sgc [ Mon Oct 05, 2009 10:09 pm ]
Post subject: 
meeni wrote:
the 4 pots have shocking pedals with standard m'c's


Utter nonsense.

I have Mini Spares 4-pot calipers on my car, and Fat Boy Dave has them on his and we have very similar pedals -- around one quarter pedal travel is necessary to get the brakes to bite. I have 5/8" rear slaves, I'm not sure what Dave has (either 15/16" or 5/8") but his pedal is, if anything, slightly better than mine (or it was when I last tried them :P)

We both have the standard (0.7" I think) brake master cylinder.

With my setup, it took a few days post installation to get a decent brake pedal. I don't remember who suggested it but they were spot on: With new calipers and piston seals, the seals drag the pistons back off the pads once you relieve brake pressure, meaning you need to move more fluid on the next application to get the brakes to bite. Also, it took two days for all the air in the system to percolate up out of the nooks and crannies before it would bleed properly.

You WILL get a decent brake pedal with 4-pot calipers and the standard brake master cylinder. It WILL NOT be as high as standard Cooper S calipers, since the same master cylinder has to move more fluid to get the pads to move the same distance. If that's not good enough for you, you will need a bigger master cylinder. Some have used clutch masters to good effect, personally I don't see the need to fart around with it as the standard master works well enough.
Author:  Convertible Mini [ Tue Oct 06, 2009 6:06 am ]
Post subject:  4 Pots
I have Metro 4 Pots on MINNIE. It has the best brakes I have ever had on a Mini. I used a late Dual Circiut master. Front feeds into remote Booster VH40 (as used on the old EH Holdens) and has very a good pedal.
If yo sneeze while holding your foot on the pedal I can assure you that you will go through the Windscreene. Make sure you mount the Calipers with the bleed nipples upwards.
Author:  meeni [ Tue Oct 06, 2009 9:43 am ]
Post subject: 
i wouldnt say utter nonsense seeing as im talking from my own expierience, i had metro 4 pots on my old race car. they stopped well but the pedal compared to cooper s discs was a little bit un nerving, this was using a standard dual circuit master cylinder and being a race car they were bled frequently so i cant say i had air in the system..
Author:  sgc [ Tue Oct 06, 2009 11:06 am ]
Post subject: 
meeni wrote:
i wouldnt say utter nonsense seeing as im talking from my own expierience


Then it would be best to share that experience, rather than just saying "the 4 pots have shocking pedals with standard m'c's" when that clearly is not true given that you also say "they stopped well".

The pedal height with these calipers and a standard MC is not "shocking".. it's a little lower than with 2-pot calipers but (at least on a road car) perfectly adequate and entirely safe.

Let's just leave the hyperbole at the door, eh? ;)
Author:  sports850 [ Tue Oct 06, 2009 1:29 pm ]
Post subject:  Re: Best master cylinder for 4 pot brakes? 0.70? 0.75?
MattE wrote:
Unfortunately the 0.75" master cylinder has decided to start leaking


I've been searching today for a master cylinder kit for a 0.75" master cylinder and can't find one so I'd suggest it might be time to switch to a 0.70" one or later dual circuit like others are using for the sake of convenience of parts .
Author:  meeni [ Tue Oct 06, 2009 3:22 pm ]
Post subject: 
sgc wrote:
meeni wrote:
i wouldnt say utter nonsense seeing as im talking from my own expierience


Then it would be best to share that experience, rather than just saying "the 4 pots have shocking pedals with standard m'c's" when that clearly is not true given that you also say "they stopped well".

The pedal height with these calipers and a standard MC is not "shocking".. it's a little lower than with 2-pot calipers but (at least on a road car) perfectly adequate and entirely safe.

Let's just leave the hyperbole at the door, eh? ;)


theres a difference between pedal movement and braking performance... so they can still stop well with a "more then normal amount of pedal travel", as your still moving fluid.. you just have to push it further for the same response... i guess its personal preference.
Author:  awdmoke [ Tue Oct 06, 2009 3:39 pm ]
Post subject:  Re: Best master cylinder for 4 pot brakes? 0.70? 0.75?
MattE wrote:
Nor have I been able to get a very good pedal with the bleeding I've done the old fashioned way.


MattE wrote:
Unfortunately the 0.75" master cylinder has decided to start leaking


I think you've found the reason for your poor pedal then.
Going from a .75" to a .7" master will mean more pedal travel, but less effort. Stick with the .75" master. A 7/8" master will give a very hard pedal unless you are running a booster.
Author:  awdmoke [ Tue Oct 06, 2009 4:26 pm ]
Post subject: 
Just worked the math, a 7/8" needs 36% more leg power than a .75"
but of course the travel is also reduced by the same amount.
Author:  MattE [ Tue Oct 06, 2009 7:55 pm ]
Post subject: 
Thanks to all for the replies so far!
I made a mistake by saying that the current brake master is 0.75. In fact it is 0.70.
It's a shame that the 0.75" brake m/cyls and kits seem to be fading from existence, as I'd quite like to try that slightly bigger size.

Can anyone comment on the merits of a pressure bleed by the local brake shop?

Cheers
Matt
Author:  sgc [ Tue Oct 06, 2009 8:01 pm ]
Post subject: 
MattE wrote:
Can anyone comment on the merits of a pressure bleed by the local brake shop?


Good idea if you can't do it yourself, but bleeding brakes ain't rocket surgery and if the master cylinder, brake lines, hoses, pistons and slave cylinders are all in good condition a pressure bleed won't work any better than a careful manual bleed.

If you don't have a helper, it'll be easier giving it to someone else to do unless you have an Eezybleed kit or similar.
Author:  92 [ Tue Oct 06, 2009 8:54 pm ]
Post subject: 
New calipers some times take a while for the seals to stop pulling the pistons away from the disc an excessive amount and this gives poor pedal travel but comes good after a short while. Usually if you work the brakes hard and get them hot they are fine.

Pressure bleeding as has been said is no real different than manual bleed just easier/quicker.
Author:  GT mowog [ Wed Oct 07, 2009 7:19 pm ]
Post subject: 
On the Early Metros (pre 1983) they had the following;-

Master Cylinder Bore 0.81" (20.6mm)
Pedal Ratio I don't recall but have checked against standard mini. It is a very slightly higher ratio (more leverage)
Booster None Fitted (later ones did, same M/C Bore but different M/C)
Front Calipers 4 spot split system (they were all the same pot size)
Rear Wheel Cylinder 0.625"
Rear Pressure Limiting Valve None fitted (latter one had these fitted)
Rear Brakes Same as standard Mini

I would suggest using the 0.75" M/C as a minimum.

Regarding the way that these are fitted to Mini's and the trouble that most seem to have bleeding them, be aware that the Calipers have 3 bleed nipples each. I have seen many of the calipers with the nipples broken off and in one case, welded up! They are easy to break off as they are only small at 7 mm - the Cooper S and 8.4" twin pots had 3/8" (just under 10mm).

I also see that when fitted to Mini's they are usually incorrectly plumbed up and this would also make them difficult to bleed. The Front Calipers have 2 seperate circuits which are designed to be connected to each circuit of a split system, not have them connected together with a short hose on the back of the caliper. Even the Works Team of the 1990's had trouble bleeding the brakes and in their operation until they seperated them. They can be made to work connected this way, but you'll always have trouble bleeding them from dry.

Also, most conversion kits I have seen have the wrong fittings for the Calipers. They have banjo fittings and copper washers, trouble is the calipers have not been spot faced and so the area that the copper sealing washer sitts on is only a rough casting! Very poor engineering. The correct fitting seals on it's inner seat.

There is a particular sequence that they should be bleed, it's the one that I've alwayed used on the Metro and had no trouble at all. I'll scan it and post here in the next day or so if someone can tell me how to attach it to my posting!
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