Brake, Coolant & Hydraulic Fluid

At the manufacturer's interval, which is typically expressed in years rather than mileage because the fluid absorbs moisture from the air whether the vehicle is used or not. That absorbed water lowers the boiling point steadily, and the consequence appears exactly when you least want it: a pedal that goes long or soft after repeated heavy braking, because the fluid has boiled and vapour compresses where liquid does not. Testing fluid moisture content is quick and settles the question.

Because the alternative is worse. A system will admit some moisture over time through seals and hoses regardless, and a fluid that repels water would let it pool at low points, where it corrodes the bores and freezes in winter. A hygroscopic fluid holds that water dispersed throughout, which protects the system at the cost of a falling boiling point. That trade is why the fluid has a service life and why a sealed container matters when you buy it.

No, and this causes more damage than most people expect. Coolants use different corrosion inhibitor chemistries, and mixing incompatible types can cause the additives to react and drop solids out of solution, which block the narrow passages in a radiator, a heater core or a cooling plate. The result is overheating from an internal blockage that no amount of new coolant fixes. If you do not know what is in the system, have it flushed and refilled with the correct product rather than topping up.

It describes the type of corrosion inhibitor. Older coolants used silicates and phosphates, which protect quickly and deplete quickly. Organic acid inhibitors work more slowly at first and last far longer, which is why long life coolants with change intervals measured in years are almost all of this type. What matters practically is that the inhibitor chemistry is what makes coolants incompatible with one another, so the technology is the reason for matching the specification rather than a marketing distinction.

Two things, and both are avoidable. Freezing protection falls, so a cold spell can freeze coolant in the system and split a radiator or a block. And the inhibitor concentration falls with it, so corrosion protection weakens even in mild weather. Mixed too strong is also a mistake, since a very high concentration transfers heat less effectively than the correct ratio. Follow the stated mixing ratio for your climate, and use deionised water rather than tap water where a concentrate is diluted.

Both sit at the low end of the range, with brake fluid supplied in a one litre presentation and coolant in a larger drum, so the drum shows a higher figure for a much larger quantity. The live minimum and maximum are shown in the band at the top of this page. Given that both fluids protect systems whose components cost many times more, and that both are consumed on a schedule, this is not a category in which to economise.

Better not to. Once opened, brake fluid begins absorbing moisture from the air in the bottle, so a container that has stood half full in a garage for a year may already be below the specification you are trying to restore. Buy the quantity the job needs, open it at the point of use, and dispose of what remains rather than storing it. The saving from using an old bottle is trivial against the point of changing the fluid at all.

More than the system capacity, because a proper bleed pushes fresh fluid through until what emerges is clean, and the quantity depends on the vehicle, the number of circuits and whether a scan tool bleed cycles the hydraulic unit. For brakes, allow well over a litre on most vehicles. For coolant, the system capacity plus a margin for the bleed and for topping up as air works its way out over the following days. Ordering short costs a second delivery.

Yes, and arguably more attentively. Regenerative braking means the friction system is used less, so the fluid gets less thermal cycling and the intervals are still time based because moisture absorption continues regardless. Brake by wire units also contain fine clearances and a high pressure pump that tolerate contaminated fluid poorly. A vehicle whose brakes are barely used still needs its fluid changed on schedule, and the low usage is not a reason to extend the interval.

Its electrical conductivity is part of the specification. Loops that pass through or alongside high voltage components must use a low conductivity coolant, and a conventional product of the wrong type in that position can lower insulation resistance and set a fault that disables the vehicle. This is not a subtle effect that appears years later. Use exactly what the manufacturer specifies, from a sealed container, and never top up such a loop with a general purpose product.

Through a proper waste route rather than a drain. Brake fluid and coolant are both harmful, coolant is notoriously attractive to animals and dangerous if ingested, and both are regulated wastes in most jurisdictions. Local authority household waste sites and garages generally accept them. Keep used fluid in a sealed labelled container in the meantime, and clean up spills promptly rather than leaving coolant on a driveway where a pet might find it.

Yes. Fluids behave differently from hardware here, because the same specification is sold in several pack sizes and only some of those have a standing price against them. State the specification you need and the volume the job takes, and a figure comes back with the shipping arrangements, since brake fluid and coolant concentrate travel under transport rules that affect the delivered cost. Confirm whether you want concentrate or ready mixed before you send the enquiry. Buying concentrate and diluting with the wrong water is the most common way an otherwise correct coolant order goes wrong.