The car servicing that can't wait, and the kind that can

Four things on a car will either strand you, cost you an engine, or fail to stop you at the worst possible moment: brake pads, brake fluid, coolant and the timing belt if your engine has one. Everything else on the service menu is negotiable in the short term. Those four aren't.

That's the whole hierarchy. Most people get it backwards, replacing a cabin air filter every year while running brake fluid that went into the car when it was new.

Brakes are the one you can inspect yourself

Pull a wheel, or look through the spokes with a flashlight if the gaps are big enough. You're looking at the pad, the flat block of friction material pressed against the disc.

New pads carry roughly 10 to 12 millimeters of friction material. Replace them at around 3. That's not a rule someone invented to sell parts; below that the backing plate gets close enough to the rotor that one hard stop can put steel on steel, and then you're buying rotors too.

Most pads have a small metal tab called a wear indicator that contacts the rotor near the end of the pad's life and makes a high, constant squeal when you're off the brake pedal. If you hear that, you have weeks, not months.

A grinding sound under braking is different. That's metal on metal, it's happening now, and it's an immediate stop-driving-and-book-it problem.

Check the rotors while you're there. Run a fingernail across the face. A lip at the outer edge means material has worn away from the middle. Every rotor has a minimum thickness stamped on the hat, the center part around the wheel studs. A shop can measure it in about thirty seconds with a micrometer, and you're entitled to ask for the number.

What the pedal is telling you

Pulsation through the pedal on a steady stop usually means uneven deposits on the rotor face or a rotor that's no longer running true. It gets worse, not better.

Pulling to one side under braking often means a sticking caliper or seized slide pins. That side is doing more of the work and wearing faster, and a caliper that's dragging all the time will cook the pad, boil the fluid and eat your fuel economy.

A pedal that sinks slowly while you're stopped at a light points at the master cylinder. That's a genuine safety fault.

A soft, long, spongy pedal means air or moisture in the system. Which brings us to the service nobody books.

Brake fluid is the most-deferred fluid in the world

Brake fluid is hygroscopic. It pulls moisture out of the air, through the reservoir cap, through microscopic paths in the flexible hoses, slowly and constantly, whether you drive the car or not.

Water in the system does two things. It corrodes the inside of the lines, the calipers and the ABS module, which is one of the more expensive assemblies on a modern car to replace. And it drops the boiling point of the fluid substantially. DOT 4 fluid with a dry boiling point well over 440°F can fall toward the 310°F region once it's saturated.

You'll never notice on a supermarket run. You'll notice descending a long mountain grade with a full car, when the fluid boils, the vapor compresses where liquid wouldn't, and the pedal goes to the floor.

Manufacturers commonly specify a brake fluid change every two years regardless of mileage. Check yours. It's a cheap service, it takes under an hour, and it's the single best value item on this entire list.

Don't mix fluid types blindly. DOT 3, 4 and 5.1 are glycol-based and generally compatible; DOT 5 is silicone and is not. Use whatever the cap and the manual say.

Coolant isn't lifetime either

The corrosion inhibitors in coolant deplete. That's the whole point of them; they sacrifice themselves so your water pump, radiator, heater core and head gasket don't.

Old coolant turns acidic and murky. You can see it. Pop the cap on a cold engine and look at the expansion tank. Clean coolant is bright and translucent. If it looks like weak coffee, or there's a brown sludge line inside the tank, it's overdue.

Long-life coolants genuinely last a long time, often five to ten years on the first fill. They don't last forever, and the second interval is usually shorter than the first. Use the exact specification the manufacturer lists. Coolant chemistries are not interchangeable, and mixing incompatible types can gel the whole system.

The timing belt is the one that ends the car

If your engine uses a rubber timing belt rather than a chain, it has a replacement interval in years and miles, and that interval is not a suggestion.

On an interference engine, which most modern ones are, a snapped belt lets the valves and pistons occupy the same space. The engine is finished. The repair usually costs more than the vehicle is worth.

A timing belt job isn't cheap, because the labor to get in there is significant. Do the water pump and tensioner at the same time while everything's apart. Paying that labor twice is how people end up angry.

Find out today whether your engine has a belt or a chain, and if it's a belt, find the interval and the date of the last change. It's a five-minute question and it's the highest-stakes one in this piece.

Transmission fluid and the "lifetime" lie

"Lifetime fluid" means the lifetime of the warranty.

Automatic transmission fluid shears down, loses its friction characteristics and picks up clutch material. A transmission that shifts hard, hunts between gears or shudders at light throttle is often telling you about fluid.

There's a real trade-off here. On a high-mileage transmission that's never been serviced, some technicians will argue against a high-pressure flush, on the theory that it disturbs deposits in a unit that's already worn. A drain-and-fill, repeated a couple of times over a few months, is the gentler route. Talk to someone who works on your specific transmission and ask which they'd do and why.

Manual gearboxes, differentials and transfer cases also take fluid, and on a truck that tows or sees water crossings, those intervals get much shorter.

What you can genuinely leave

Cabin air filter. It affects how the car smells and how well the air conditioning blows. Change it when it bothers you.

Fuel injector cleaning, induction services and engine flushes offered at the counter. Sometimes justified, often not, rarely urgent.

Wiper blades until they streak. Spark plugs until close to their interval, which on modern iridium plugs can be a very long way out. Cosmetic work, forever.

Tires are their own category, because they age out as well as wear out. The four-digit DOT code on the sidewall gives the week and year of manufacture. Rubber that's eight or ten years old is hardening whatever the tread depth says.

How to talk to a shop

Three questions cover almost everything.

• "Can you show me?" A good shop will walk you to the car.
• "What's the measurement?" Pad thickness in millimeters, rotor thickness against minimum, fluid moisture content as a percentage.
• "What happens if I wait three months?" This is the important one. The honest answer sorts the safety items from the revenue items, and you'll learn a lot about the shop from how they respond.

Write the date and mileage of every fluid change in a notebook in the glovebox. Not an app. A notebook, in the car.

Three years from now, when you're standing in a service department being told your brake fluid has never been done, you'll be able to open the glovebox and settle it in ten seconds.