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 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.
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.