25 replies to this topic

[Ethel]

It really doesn't matter that much if it's one bore all the way through. If you're using the stepped bore, GMC227 cylinder (yellow band & odd size metric unions) then the rears go on the bottom.

[mini-geek]

yep as Ethel says the rear is the bottom on a yellow tag MC like mine.

[TopCatCustom]

Does it use a stepped piston that completely seperates the front and rears then? If so thats why I wasn't sure about using a regulator in one line, i.e. if you limit the pressure then the pedal goes harder without travelling so far, does it not prevent full pressure being exerted on the fronts which are mechanically linked to the same piston on the MC?

Does that make any sense at all and does anyone know where I'm coming from??!

[Cooperman]

You could use the later dual-line cylinder and servo, as the late cars. Then remove that nasty metric-threaded pressure balance valve on the front bulkhead, run the rear brake line inside the car and use an early-type but adjustable pressure valve as on early cars on the sub-frame.
One might think that you could actually mount the valve inside the car under the rear seat panel so that it is not adjustable by the driver whilst seated in the car. In fact, the MoT tester would not see it if it was mounted behind a drop-down panel screwed in place and that would probably then meet the regulations.
Personally I like a servo on a rally car and when I tried my 'S' without one I found that they lacked feel and then tended to grab. That was with carbon-metallic race pads and 7.5" discs in the wet. In fact, whilst testing it I nearly spun it! Re-fitted the servo straight after I got back to the workshop.

[TopCatCustom]

Hmm now I have a servo dilema!

[dfmini]

If so thats why I wasn't sure about using a regulator in one line, i.e. if you limit the pressure then the pedal goes harder without travelling so far, does it not prevent full pressure being exerted on the fronts which are mechanically linked to the same piston on the MC?

Good remark.
That mean you regulate the pressure in all the system (front and rear). Thinking this way, it sounds scary.

As people tried that system already, i guess something must be wrong with your reflection, but i cannot see what

[hustybusty]

so could you just use a T-piece from a single MC and have a pressure regulator along the rear supply?

[Ethel]

Yes, that's how it was done up to '76, but as above, cars are supposed to have dual circuit brakes from then on.

Regarding dual masters - there are 2 pistons in the same bore with fluid in between them (connected to one of the circuits). If all's well, the brake linings will limit the travel at the wheels and keep enough fluid between the pistons to hold them apart & there will be equal force on either side of the upper piston and it'll behave like a single piston cylinder that just happens to have 2 outlets. If you have a yellow band cylinder, the upper piston has a smaller diameter on top than below, the forces on the piston are still equal but the upper cylinder is at a higher pressure by virtue of the smaller piston area.

The higher pressure gives more power to the front brakes. If you hooked the circuits up the wrong way round there'd be more pressure in the rear line for the under pressured front line to cut off in the brake limiter valve = spinning Mini.

If you're rallying it'd be well worth considering an inertia valve to control the brake balance - they work by the weight transfer to the front rather than line pressure, so compensate for going up 'n down hill as well as road grip.

[TopCatCustom]

If there is no phsyical link between the top and bottom piston as Ethel says (linked by fluid) then it would make perfect sense, if they were linked by a rod then my theory of a restriction in one line (or indeed non adjusted drums) would prevent getting good pressure in both circuits.

I guess the yellow tag type must have a spring or something in between to softly link pistons so you can bleed them up etc, with 4 pot front calipers would I not be better off with equal sized pistons as the extra area in the calipers should provide the extra pressure, and a smaller master piston would increase pedal travel (especially with 4 pots) would it not? If this is the case, what type could I look for which doesn't have biased sizes?

Also I'm not sure on the inertia valve, they seem like a good idea but if they were that good then nobody would have manual bias valves, and about every rally car I've seen has a manual one, and I do see them every day!

[Cooperman]

Interestingly, the brake system on my 1973 Innocenti 1300 is a dual line system. However, it hs a Mk3-type servo onthe fronts only and a pressure relief valve for the rear, although it's not a Mk.1 type on the rear sub-frame, but one which is mounted on the bulkhead. Seems to work though.

[Ethel]

The Yellow band is the same size as the others on top & makes the same pressure for the front end, the bigger bottom reduces pedal travel by not providing more pressure to the rear than is needed. You can achieve the same with smaller rear wheel cylinders - Metro line pressures are pretty much identical to Minis (single bore size master, smaller rear cylinders). All masters have return springs: the pistons aren't physically attached to the pedal so need something to return them to their rest positions.