When Should brake fluid and coolant be renewed?

The most important quality of brake fluid is its boiling point (about five times higher than the boiling point of water). The higher the better, so even in severe use, it does not vaporise or become aerated and lose some of its force in the hydraulic pressure system that operates the brakes. After all, braking hard from, say, 100 kilometres per hour to a near standstill even once, the friction pads on the discs generate enough heat to boil a litre of water. Brake discs can glow red hot, and the fluid has an interface with that.

Brake fluid should also contain nothing that will allow corrosion. Rust on the hydraulic pistons can make them jam and/or progressively erode the metal until they leak and lose pressure.

Brake fluid “ages” through a decline in these two respects, mainly because it is hydroscopic, it absorbs water from the atmosphere. Even though the system is theoretically sealed, inevitably, some water (even humidity in the atmosphere) gets in when the reservoir cap is opened or round the pistons at the wheel hub, which move every time you touch the brake pedal.

Even tiny quantities of water can accumulate, not only allowing corrosion but actually promoting it, and the water element has a low boiling point in brake-temperature terms, so it aerates the hydraulic fluid and compromises its physical integrity. The brakes get “spongey” and the boiling point of the fluid itself is halved when it is fully saturated with water.

 So, here is the regimen

The brake fluid level should be checked whenever the bonnet is opened. It should be kept topped up to above the MIN line (and preferably up to the MAX line) at all times. Even if all is well, the level will fall slightly as the brake pads/linings wear thinner and need topping up until the brake pads/shoes are replaced.

Brake fluid is a completely clear liquid (no colour, no cloudiness). Water content makes it turn cloudy (a sign that water has mixed in) and become progressively brown (that is a result of rust, and after extreme neglect it can turn almost black). At the first signs of it becoming anything but colourless and clear, it should be changed, completely drained out (flushed if necessary), the brake pistons should also be derusted and the system refilled, bled of all air, and firmly sealed.

Takeaways

The coolant for your engine is essentially water, preferably with an additive (some form of glycol such as anti-freeze) that will prevent corrosion and help lubricate the water pump.

The water pump is a sealed unit at the front of the engine, driven by the fan belt. It forces the coolant to circulate in its constant cycle from the bottom of the radiator into the “water jacket” channels of the engine block (where it draws heat from the engine). Then, it goes back through pipes to the top of the radiator, from where it flows through the finned radiator tubes (cooled by air entering the engine compartment and boosted by the fan), before returning to the bottom tank and beginning the cycle again.

For the system to work properly, there must be enough coolant (filled up to the housing of the radiator cap), it must flow vigorously and rapidly through the whole cycle and must be well sealed and under pressure to prevent air bubbles, recapture steam and raise the boiling point. It must be adequately cooled by lots of fresh air entering the engine compartment and boosted by the fan when necessary. If any of those ingredients is not present and correct, then some degree of overheating, whether slight or damagingly severe, is inevitable.

Overheating

One of the most common causes of overheating, especially in older cars, is rust and other dirt in the system. This reduces the rate of cooling in the radiator tubes (it compromises the ability of the radiator fins to conduct heat away) and on the inside reduces the flow of coolant in the water jacket channels around the engine, allowing the coolant to spend too long in the walls of the engine block and become super-heated (and hence bubbly and steamy).

This problem is best prevented by regularly flushing the system with clean water and then refilling it with fresh water and a good quality coolant additive. It is cured by flushing with special fluids to remove the dirt/rust from the radiator tubes and water jacket channels. In severe cases, these might need to be physically scrubbed, inside and out and then flushed again. Dirt on the outside of the radiator fins (grass seeds or mud) also needs to be promptly removed, but carefully, as they are delicate material to optimise the conduction of heat.

As with brake fluid, discolouration of coolant is a sign that it is time to start afresh.

What else goes wrong?

If the fan belt breaks, or the water pump itself fails, circulation of coolant will stop and the engine will overheat almost immediately.

If any part of the system leaks (the tubes or the tanks of the radiator or the tubes connecting it to the engine), then coolant will be lost and the remaining volume will not be enough to absorb the engine heat and take it away for fan cooling.

If the radiator cap spring is too weak, it will allow coolant to escape too easily and be lost to the outside world. It will also reduce the pressure of the sealed system, so the coolant will be weakened by air bubbles and steam.

The system also has a gadget called a thermostat (usually in a housing at the front of the cylinder head) whose job is to keep the engine temperature constant.

When the engine is cold at first starting, the thermostat closes and shuts off coolant circulation so the engine warms up as quickly as possible. When the engine reaches its designed operating temperature, the thermostat partially opens a valve and the cooling circulation begins.

If the engine is working hard and generating more heat, the valve opens wider, if the engine generates less heat (gentle driving or very cold weather), the valve opening is reduced.