Brake Fluid for the AC Aceca: Types, Checking, Choosing and Changing

What brake fluid does, and why it matters

Brake fluid is the incompressible link between your foot and the brakes: press the pedal and the fluid transmits that force to the cylinders at each wheel. For that to work it must stay liquid and incompressible, it must never compress or boil. Its enemies are heat and, for most types, water.

Over time ordinary fluid absorbs moisture from the air, and water lowers its boiling point. So under hard or repeated braking, a long descent, a spirited run, a warm queue at a show, old fluid can boil, putting compressible vapour in the lines. That is brake fade: a soft or sinking pedal exactly when you need the brakes most. It is why fluid is a service item, not fill-and-forget.

The Aceca uses conventional brake fluid, not mineral fluid

First, clear up a common worry. The Aceca has a conventional Girling hydraulic system, so it uses conventional brake fluid, glycol or silicone. It does not use the mineral fluid (LHM) found in some Citroëns and in certain Rolls-Royce and Bentley systems. Mineral and conventional fluids are completely incompatible and must never be confused. If you have seen mineral fluid mentioned, it does not apply to your Aceca.

The two families: glycol and silicone

There are two families of conventional brake fluid, and the golden rule is that they must never be mixed.

• Glycol (DOT 3, DOT 4, DOT 5.1): the traditional type, amber or pale straw when fresh. It is the modern heir to the original Girling fluid the Aceca left the factory with. DOT 4 is the usual choice today: readily available, a higher boiling point than DOT 3, and what most owners, and AC Owners' Club members, recommend. DOT 5.1 is a higher-specification glycol that is also compatible with DOT 4. The downsides: glycol absorbs moisture, so it needs renewing every one to two years, and it strips paint, so wipe spills at once.
• Silicone (DOT 5): the odd one out, and note the confusing numbering: DOT 5 is silicone, but DOT 5.1 is glycol. Dyed purple, it does not absorb moisture and does not harm paint, which is why it is popular on restored cars and cars that spend long periods standing. The downsides: some owners report a slightly softer pedal; because it does not absorb water, any moisture that does get in tends to pool at low points such as the calipers rather than dispersing; and it is more expensive and less widely stocked.

The one rule that matters most: never mix them

Glycol and silicone must never be mixed. Putting silicone into a system that has had glycol, or the reverse, without a complete strip, flush and ideally new seals and hoses lets the two react: gelling, sludge, swollen seals and a spongy, unreliable pedal. That is a safety failure, not just a nuisance.

The practical consequence is simple: the moment you do not know what is in the system, glycol DOT 4 is the safe default, because a clean changeover to silicone is a job for new or known-clean components, not a top-up. It is telling that experienced AC Owners' Club members who favour silicone use it only on cars restored with all-new pipes, hoses and seals; their advice is that if there is any chance of old fluid in the system, DOT 4 is the way to go.

Which is in your Aceca? How to tell

Because you must match what is already there, identifying it comes before anything else. Three checks:

• Colour in the reservoir: amber or pale brown means glycol; purple or violet means silicone. A phone photo or a drop on a clean white rag makes it easier to judge.
• The master-cylinder cap: it is often stamped with the DOT specification.
• The history file: a silicone conversion during restoration is usually documented in the invoices, and a thorough restoration with new pipes, hoses and seals is exactly when silicone tends to be fitted.

If it is genuinely unclear, a brake specialist or whoever recommissioned the car can confirm it. When in doubt, do not guess and do not top up, find out first.

Checking your fluid before a run

This is the quick habit worth doing before any drive, quite different from a full change. You are looking at four things:

• Level: the master-cylinder reservoir should sit between its minimum and maximum marks. A little drop over time is normal as disc pads wear and the calipers take up more fluid; a real drop means a leak, find it before you drive.
• Colour and condition: fresh fluid is clear and pale; dark, murky fluid is old and moisture-laden (on glycol) and due a change.
• Pedal feel: firm and high, not soft or sinking. A pedal that goes long, soft, or sinks slowly under steady pressure is a stop-and-investigate sign.
• A look around: no weeping at the flexible hoses, unions, wheel cylinders or calipers or the master cylinder, and no fluid on the inside face of a wheel.

What to keep on hand is a small, sealed bottle of the correct fluid for your car. DOT 4 if it is on glycol, DOT 5 silicone only if that is what it runs, for a top-up. Buy it small and reseal it, because opened glycol starts absorbing moisture. But remember: a genuine drop in level is a fault to fix, not simply a top-up.

Disc-braked cars run the fluid hotter

Front disc brakes were an option on the Aceca from 1957 and standard from 1958. On a disc-braked car the fluid at the calipers runs hotter than in an all-drum car, so a high, stable boiling point matters more, another reason to keep glycol fresh, or to count silicone's stable boiling point as a point in its favour. The fluid type is not changed by having discs; it is the freshness that matters.

The clutch shares the fluid

On the Aceca the hydraulic clutch draws on the same fluid family as the brakes. So whatever the brakes run, the clutch runs too, and if you ever convert one to silicone you must convert both, or you will be storing two incompatible fluids and inviting a mix-up.

When and how to change it

Glycol fluid should be renewed every one to two years regardless of mileage, because it absorbs moisture continuously; silicone is more stable but still benefits from periodic renewal. A full change also flushes out sediment and restores a firm pedal. The change itself, flushing the old fluid through and bleeding the air out, working from the wheel furthest from the master cylinder first, is a straightforward two-person job, set out step by step in the brake-fluid service guide.

If the pedal stays soft after a proper bleed, stop. That points to a system fault, a leak, a failing master cylinder, or a perished flexible hose, and it is specialist territory, not something to chase with more bleeding.

In short

The Aceca uses conventional fluid, not mineral. Glycol DOT 4 is the safe, available default and the heir to the original Girling specification; silicone DOT 5 is a paint-friendly, moisture-resistant alternative that suits freshly restored cars and cars that stand, but the two must never be mixed, so identify what your car runs before you touch it. Check the level, colour and pedal before a run, keep glycol fresh, and treat anything soft or low as a reason to stop and investigate.

Sources. This article draws on the AC and Bristol workshop manuals and the experience shared by owners in the AC Owners' Club community.

Brakes are safety-critical and this is a general guide, not a substitute for the workshop manual or a brake specialist. Identify the fluid your car runs before adding any, never mix glycol and silicone, and if the brakes feel soft, low or wrong, do not drive the car, have it checked.