In the eighth in­stal­ment of our han­dling se­ries, Neil gives us the low­down on brakes, this time look­ing at hy­draulics and fluid.

In the eighth in­stal­ment of our han­dling se­ries, Neil is once again talk­ing about brakes, or more im­por­tantly, the fluid and hy­draulics.

Mini Magazine - - Contents -

Mini brakes – in­deed all mod­ern brakes – are a hy­draulic sys­tem. They use the fact that you can’t com­press a liq­uid to trans­fer force from your pedal op­er­at­ing the mas­ter cylin­der to the brak­ing sur­faces via the slave cylin­der.

Brak­ing fluid comes in four com­mon types, DOT 3, DOT 4, DOT 5 and DOT 5.1. Most are a gly­col-ether for­mu­la­tion. This is the tra­di­tional type of fluid and is com­monly avail­able but has draw­backs, firstly it is hy­gro­scopic and sec­ondly it is cor­ro­sive. DOT 5 is a sil­i­cone-based for­mula and 5.1 is again gly­col-ether.

DOT 3 has a boil­ing point lower than DOT 4. DOT 4, lower than DOT 5.1 and the high­est is DOT 5. The higher the boil­ing point, the hot­ter the brakes can get be­fore the fluid starts boil­ing and you lose the feel in the pedal, this is dif­fer­ent to brake fade when the pads go be­yond their func­tional tem­per­a­ture.

The prob­lem with a hy­gro­scopic fluid is that it ab­sorbs wa­ter from the at­mos­phere, which is why it’s rec­om­mended that brake fluid is changed on a reg­u­lar ba­sis.

Chang­ing a Mini to sil­i­cone fluid is pos­si­ble, where mod­ern cars with ABS can have is­sues. You should how­ever en­sure that your sys­tem is fully drained of the old fluid and some rec­om­mend chang­ing all the seals at the same time. The is­sue with sil­i­cone fluid is that if any wa­ter is present in the sys­tem then it will gather at the low­est point and po­ten­tially cor­rode the area it’s sit­ting in.


There are three ba­sic for­mats for the Mini brak­ing sys­tem. Sin­gle line, di­ag­o­nal split and front/ back split.

With the sin­gle line sys­tem, the mas­ter cylin­der has one pis­ton and a sin­gle pipe ex­its. This then splits to go to each front brake and the rear of the car. At the rear, there is a split­ter which lim­its the pres­sure which can reach the back brakes for bal­anc­ing rea­sons and goes to each rear wheel from there.

The sin­gle line sys­tem fails if any

“The prob­lem with a hy­gro­scopic fluid is that it ab­sorbs wa­ter from the at­mos­phere”

pipe splits or cylin­der leaks. A safer way to do this is to split the brak­ing hy­draulics into two parts so you should have half of the sys­tem work­ing if there

“The sin­gle line sys­tem fails if any pipe splits or cylin­der leaks”

is a prob­lem. The first way of split­ting the sys­tem was di­ag­o­nally. This means that if you spring a leak, you al­ways have one front and one rear brake work­ing. This makes a lot of sense in some ways how­ever it would pull the steer­ing away from the work­ing front brake, which is okay at slow speeds with drum brakes, but if you are at speed with ef­fi­cient discs, then it may take you by sur­prise.

The later Minis were fit­ted with a front / back split. The rear brakes are on a dif­fer­ent hy­draulic cir­cuit to the front brakes. With this sys­tem you are more for­tu­nate if it’s the rear brakes which fail as the front brakes do more work, but cor­rectly ad­justed rear brakes can get you to safety. As there is no hy­draulic con­nec­tion be­tween the front and rear brakes, then there is a valve which will limit the rear pres­sure flow de­pend­ing on the front cir­cuit pres­sure.


Why does it mat­ter how the pres­sure is dis­trib­uted be­tween the front and back? It’s down to bal­ance, the same as most of the setup. Your grip is all gov­erned by the tyres. When you are car­ry­ing out

max­i­mum brak­ing in a straight line, you want to use as much of that grip as pos­si­ble on each of those four tyres to slow you down. If your back tyres are do­ing less work than the front tyres, then you are not get­ting the most your of your rear tyres. If your back wheels lock up be­fore the fronts, then you can’t use all of your front grip.

The back brakes do less work due to the phe­nom­e­non of weight trans­fer­ence. When you brake, then the ve­hi­cle weight trans­fers on to the front wheels and the back gets lighter. With more weight on the front, the rub­ber is be­ing forced into the road more and so you can ap­ply more stop­ping force. It’s this ef­fect which is the rea­son we can have less ef­fi­cient brakes on the rear, as they are do­ing less work. It then fol­lows that we need less hy­draulic pres­sure on the rear so that the brakes are not ap­plied as hard as those at the front.

Hav­ing bal­anced brakes makes for a much more sta­ble car un­der brak­ing, which then in­spires con­fi­dence as well as mak­ing the most of those tyres.

Words and Pho­tog­ra­phy Neil Burgess

Brak­ing sys­tems. Sin­gle line brake mas­ter cylin­der.

Bulk­head-mounted bias valve for front / back split brakes.

Servo for sin­gle line brak­ing sys­tem.

Mas­ter cylin­der for dual-cir­cuit brakes.

Dual-cir­cuit brake mas­ter in­cor­po­rated within servo.

Pres­sure reg­u­la­tor on rear sub­frame for sin­gle line brakes.

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