Brake hydraulics: bleeding, and why the pedal goes soft

The whole braking system rests on one property of liquids: they do not compress. Push on a column of fluid at one end and the far end moves immediately, by the same amount, with no give in between. That is what makes a hydraulic brake feel connected to the road rather than to a spring.

Which means the entire subject of a soft pedal has one sentence in it, and the corpus already states it at brake pedal feels soft or spongy: something compressible has got into a system designed to carry only liquid. Everything below is a way for that to happen.

The original explains the hydraulics properly and never gets to the fluid, which is where all of this lives.

The chain from your foot to the pad

The original's sequence is correct and worth keeping in order:

1. Your foot moves the pedal.
2. The servo multiplies it, using vacuum from the running engine to add most of the effort.
3. That pushes the master cylinder, which converts force into fluid pressure.
4. Pressure travels through steel pipes and flexible hoses to every wheel.
5. At each wheel a caliper piston or wheel cylinder converts it back into force on the friction material.

Step 2 carries a consequence the original does not draw, and it is the one worth knowing.

The servo stores its vacuum, and the store is small. With the engine off you have perhaps one or two assisted stops, and after that the pedal goes rock hard and needs a genuinely alarming shove for a fraction of the braking you expected. The brakes have not failed — the assistance has — but the difference is not something you discover comfortably at speed.

That is exactly the situation described in the engine cut out and will not restart, and it is why P0339 — an intermittent crankshaft signal — is treated as a serious code rather than an inconvenient one. An engine that stops on the motorway takes the steering assistance and most of the braking assistance with it.

Two circuits, not one

The original describes a single system. Every car built for decades has two independent hydraulic circuits, usually split diagonally — one front wheel paired with the opposite rear — so that a burst pipe or a failed seal costs you half the braking rather than all of it.

It is worth knowing what that failure feels like, because it is not the total loss people imagine: the pedal goes most of the way to the floor and then finds something. Long travel, weak retardation, and the car probably pulls. If you ever get that, you still have brakes; you have half of them, and you should be stopping.

Why the pedal goes soft

The fourth row is the same failure as the one in the clutch, for the same reason and with the same signature: nothing on the ground. A master cylinder seal that has given up lets fluid slip past the piston inside the bore, so the pedal sinks while the reservoir stays full and nobody finds a drip. It is the failure most likely to be dismissed as "it just needs bleeding".

And one instruction belongs with the whole of that table. The corpus treats a soft pedal as urgent and says so plainly at brake pedal feels soft or spongy: check the fluid level, and if the pedal travels a long way or keeps sinking, do not drive the car — have it recovered. That applies to the boiled-fluid row and the master-cylinder row as much as to a leak, and it is worth saying because those are precisely the two people carry on driving: one of them recovers as it cools, and the other loses nothing on the ground. A pedal that came back has not been repaired. It has been reset.

The fluid, which the original never mentions

This is the omission that matters, because brake fluid is the only fluid in the car that gets more dangerous with time rather than dirtier.

It is hygroscopic — it absorbs water from the air, continuously, through the reservoir and even through the flexible hoses. A glycol brake fluid can take up a percent or two of water a year, and it never gives it back.

Water is what ruins it, and the mechanism is the compressibility rule again. Fluid specifications quote two boiling points: dry, straight from a sealed bottle, and wet, with 3.7 per cent water in it. For DOT 4 the difference is roughly 230 °C down to about 155 °C. A brake that has been working hard on a long descent reaches those temperatures easily at the caliper.

So the failure goes like this. The fluid nearest the caliper boils. Boiling makes vapour, and vapour compresses. The pedal — which was firm all morning — goes to the floor, and it does so precisely at the bottom of the hill where you need it. Let it cool and the vapour condenses and the pedal comes back, which is why this fault is so often reported as intermittent and so often dismissed.

The rules that follow are short:

• Change it on time, not on mileage. Two years is the common interval and it is a time interval because the water gets in whether you drive or not. A car that does 3,000 km a year needs it as much as one that does 30,000.
• Use the specification the car asks for. DOT 3, DOT 4 and DOT 5.1 are all glycol-based and mix with each other; going up a grade is generally safe, down is not.
• DOT 5 is silicone and must never be mixed with any of them. The number looks like a continuation of the series and it is a different chemistry. It is for stored classics, not for a car in use.
• Keep it off the paint, and off your hands before you touch a friction surface.
• Buy small bottles and close them. An opened bottle on a shelf is absorbing water from the moment the seal is broken, so the half-litre saved from last time is not the fluid you want to put in.

Bleeding

The merged piece on bleeding turned out to be a video with no text, so this is written rather than translated.

The point of bleeding is to push every trace of air out of a system whose whole function depends on there being none. It is also how the fluid gets changed, because you are replacing the contents either way.

• Never let the reservoir run dry. If it empties, air goes into the master cylinder and you have converted a simple job into a difficult one. Check it after every wheel.
• Work in the order the manufacturer gives. The old rule of starting at the wheel furthest from the master cylinder is right often enough to be useful and wrong on plenty of diagonally split systems. Look it up.
• Do not push the pedal to the floor on an old car when bleeding by the two-person method. The master cylinder piston travels into a part of the bore it has never reached, where corrosion has built up, and it can tear the seal on the way. Put a block under the pedal to limit its travel.
• A car with ABS may need a scan tool. The modulator holds fluid in passages that a normal bleed does not reach, and the tool cycles its solenoids and pump to flush them. If the pedal is still soft after a textbook bleed on an ABS car, that is usually why rather than a fault.
• Finish with a firm pedal test before the car moves: pump it up, hold hard pressure for thirty seconds, and watch for any sink at all.

And if the fluid coming out is dark, that is the answer to when it was last done.

Two things the corpus can and cannot tell you

It can report the electrical side: the brake pressure sensor and its partner at C0045, and the pedal position sensor that tells the modules how hard you are asking.

It cannot report air in the lines, water in the fluid, or a master cylinder passing internally. None of those sets a code, exactly as pad wear does not — see brake pads and discs. The hydraulic half of a braking system is diagnosed with a pedal, a torch and a bottle of clean fluid, and a clean scan says nothing about it at all.

Video guides

Video, for the parts of this that are easier watched than read.