Im upgrading to 8.4 discs on my deluxe and was wondering if there was any benefit to upgrading to dual circuit?

If so what needs to be done....?

If I stay on the single circuit, apart from changing to 5/8th rear cylinders do I have to change anything else like the brake balance valve?

Only benefit of the dual circuit late Oz Mini M/C is it complies with the ADRs for the late 70s.
Don't kid yourself it's any safer, 13secmini's Starlet turbo Mini had no pedal at all until we fitted new rear wheel cylinders. And the front discs had been bled fine... so much for early dual circuit systems.

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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.

Cheers Kev, am more curious than anything else, mostly info from uk/us sites regarding brake circuit upgrading when moving up to the 8.4s

Technically speaking the dual circuit master cylinder is more failsafe because it has a warning light to tell you when your brakes have just failed. But hell, you have a mechanical hand brake for when it all goes to sh*t.

I wouldn't recommend going out and buying a dual circuit master cylinder as an upgrade. But if you have a spare one lying round doing nothing, and you like to tinker, I don't see why not. If it doesn't work out, just put your single circuit m/cyl back on.

Cheers Anto...



Dont think a light is really needed to let you know the brakes have failed, lol

No , but it'll give you that second or so of self confidence to think "hey , my brakes have failed so it isn't my fault I'm about to hit that tree" closely followed by "OH #$@#@" anyway ...

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The thing that makes the dual circuit a 'failsafe' is the balalnce cylinder. I have pulled apart a couple of the balance cylinders (the one under the MC, some call it a regulator) and all have been rusted solid inside. This means in the event of a leak on one circuit the thing won't move anyway making the idea of the dual circuit pointless (the pedal will still go to the floor).

AFAIK you can't buy rebuild kits either (they are sealed unit)......You can possibly buy a replacement here item 27

Whether or not the dual circuit are better is of personal opinion, I have one in my car with std discs (Cooper S) and I think the plastic res one was just as good pedal travel wise.

Daniel

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Rebuild kits are only available for the late rover types (-> GMC227).
Maybe for some others like the servo type.

Anyways, if the cylinder is worn even an rebuild kit is useless.

I feel much safer on the road with an dual circuit system.
It true that even a dual circuit system can fail... But chances of total fail are less.

If you want to upgrade; i'd use just the setup used on late pre-servo Rover Minis.

GMC227, balance-thing-valve on firewall (FAM7821), Rear/Front split.
FAM7821 replaces the Cooper S valve at the rear.
Cars with FAM7821 only had a t-piece at the rear (Part N°3H2424).

GMC227 is the type without pressure warning light switch thing.

Cheers,
Jan

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No speakers, no bodykits, no six-inch spacers, no spak filler, no bullshit

toying with the idea . using two single circuit cylinders attached to one pedal for the brakes. has to be adjustable of course. what do you think?

Why not buy a set of Wilwood race M/Cs with balance bar, they are designed for this.

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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.

$$$ and mainly because i prefer to build it myself.

I have this theory that the more things on a car that are adjustable the more chances you have of getting the adjustment wrong.

One thing to make sure of with a balance bar is that there is enough stroke left if one circuit fails. I have seen them set up that when one side fails the pedal is almost on the floor before the other side takes up.

Or if you're really flush KAD ones....

Whether you use a single circuit or dual circuit master cylinder, 8.4" discs should be used with the pressure limiting valve (FAM7821) to the rear brakes and 3/4" rear cylinders.

You can get away with smaller rear cylinders - ie. the car will still be safe, but the brake balance will be biased more to the front.

The only advantage of going to a dual circuit master cylinder is the safety one. If one circuit leaks or runs out of fluid the other curcuit still works. That's the theory anyway - I've had a leak in the rear circuit and like someone noted above, the pedal still went to the floor! That's when you pull on the hand brake, and if it's properly maintained it will stop the car surprisingly well.

The only disadvantage to a dual circuit master cylinder is the very slightly increased pedal travel before the brakes stary to come on.

BTW, contrary to popular belief FAM7821 is simply a pressure limiting valve to the rear brakes and performs the same function as the limiter which used to be installed at the rear subframe. It does not balance the pressures between front and rear, and it does not provide any safety function in the event of a leak in one circuit.

I would be wary or using front discs with 3/4" rear wheel cylinders with a dual circuit M/C and the FAM7821 valve.
I nearly swapped ends on the F2 once at 110KMH, but had no problems since replacing them with 5/8" (S) ones.
And, this was with 7.5" S brakes on the front. 8.4s should give even more forward weight transfer under heavy braking, shouldn't they??

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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.