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Why Brake Fluid Degrades in a Classic Car

Brake fluid is the one consumable that quietly saves your life every time you press the middle pedal. In a classic, it deserves even more respect. Most cars use glycol-based fluid — DOT 3, DOT 4 or DOT 5.1 — and these are hygroscopic, meaning they actively absorb moisture from the air. That moisture doesn't just sit there. It lowers the fluid's boiling point, encourages corrosion inside wheel cylinders and calipers, and can turn a firm pedal into a spongy one after a long descent. A classic that sits for months at a time is especially vulnerable, because damp air in the reservoir and lines has plenty of time to do its work. Silicone DOT 5 fluid doesn't absorb water, but it isn't a magic cure. It can trap air, give a softer pedal, and is not compatible with glycol fluids. Unless the whole system has been rebuilt for it, most classic owners are better off sticking with a good DOT 4 and changing it regularly.

Monthly Checks That Take Two Minutes

Start by parking on level ground and wiping the reservoir clean before you open it. Dirt around the cap is the enemy. Most classic reservoirs are translucent, so you can check the level without removing the lid, but it pays to look properly. The fluid should sit between the MIN and MAX marks. A gradual drop as the pads or shoes wear is normal, but a sudden drop means a leak — often a wheel cylinder, a caliper seal or a rusty pipe. Never top up and forget. If the level has fallen, find out why.

Look at the fluid's colour. Fresh glycol fluid is almost clear, with a pale straw tint. As it ages it turns amber, then dark brown or black. Dark fluid is full of moisture and corrosion particles, and it should be replaced. If you want to be scientific, a cheap electronic moisture tester or a pack of test strips will tell you the water content as a percentage. Anything above three per cent is past its best. Also check the reservoir cap and its rubber diaphragm. The diaphragm flexes as the level drops, and if it's split or perished, moist air gets straight into the fluid. A new cap seal costs pennies and is one of the best value jobs on any classic.

Choosing the Right Fluid and Knowing When to Change

For most classic road cars, a quality DOT 4 fluid is the sensible choice. It has a higher boiling point than DOT 3 and is widely available. DOT 5.1 is also glycol-based and can be used in place of DOT 4, but it absorbs moisture faster, so it needs changing more often. Do not confuse DOT 5.1 with DOT 5 — the latter is silicone and must never be mixed with glycol fluid. If you are unsure what is in the system, bleed a little out and look at it. Glycol fluid mixes with water and turns cloudy; silicone fluid separates and feels slippery.

• Change interval: every two years for a road car, or annually if you do track days, hillclimbs or live in a damp climate.
• Never reuse fluid: once a bottle is opened, it starts absorbing moisture. Buy fresh, seal it tightly and use it within a few months.
• Keep records: write the date and fluid type on a label under the bonnet. Future you will be grateful.

Bleeding the System: Tools and Sequence

Bleeding removes air from the hydraulic circuit. Air compresses, fluid doesn't, so any trapped bubble makes the pedal feel soft and reduces braking effort. You will need a flare nut spanner that fits the bleed nipples properly — usually 8mm or 10mm on classics — a clear plastic tube that fits snugly over the nipple, a clean glass jar with a little fresh fluid in the bottom, and plenty of rags. Brake fluid strips paint, so keep water nearby to rinse any spills immediately.

Work in this order: start at the wheel furthest from the master cylinder, then work closer. On a right-hand-drive car that usually means rear left, rear right, front left, front right. If your car has a dual-circuit system split front and rear, do both rears first, then both fronts. If it is split diagonally, follow the workshop manual. Before you start, adjust the rear drum brakes if your classic has them. If the shoes are far from the drums, the pedal will travel a long way even with perfect fluid. Also check that the bleed nipples are not seized. A little penetrating oil a day beforehand can save a snapped nipple.

For two-person bleeding, have your helper press the pedal down slowly and hold it. Open the nipple a quarter turn, watch the fluid and bubbles flow through the tube, then close the nipple before your helper releases the pedal. Repeat until the fluid runs clear with no bubbles. Check the reservoir after every wheel and top it up. Never let it run dry, or you will pull air back into the master cylinder and have to start again. A pressure bleeder makes the job a one-person task, but keep the pressure low — around 15 psi — to avoid foaming the fluid.

Troubleshooting: Spongy Pedal, Leaks and Rear Drums

If the pedal is still spongy after bleeding, air is probably still trapped. Try bleeding again, and check the master cylinder. If it has been allowed to run dry, it may need bench bleeding. A master cylinder that is bypassing internally will give a pedal that slowly sinks under steady pressure. Look for wet patches around the calipers, wheel cylinders, flexible hoses and the master cylinder itself. A bulging flexible hose can also cause a soft pedal under pressure.

On classics with rear drums, the adjusters are often overlooked. If the shoes are not close to the drums, the wheel cylinder has to push them further, and the pedal feels long. Adjust them before bleeding, and check the handbrake linkage too. Finally, dispose of old brake fluid responsibly at your local council recycling centre. Never pour it down the drain. With fresh fluid, a firm pedal and a careful eye on the reservoir, your classic will stop as confidently as it did when it left the factory — and that is the whole point of keeping it on the road.