Silicone DOT 5 vs DOT 4 Brake Fluid in Classic Cars
Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent
DOT 5 brake fluid in a classic car is silicone based, while DOT 4 is glycol based. Silicone does not absorb water and will not damage paint, which suits cars that sit for long periods. The catch is that the two must never be mixed, so switching means a full conversion, not a top up.
Here is how the two fluids differ, what goes wrong if they meet, and what to think about before converting.
The basic difference
Brake fluids are graded by “DOT” numbers. The number does not mean one is simply better than another. The base chemistry is what matters.
- DOT 3, DOT 4 and DOT 5.1 are glycol based.
- DOT 5 is silicone based.
DOT 5.1 sounds like a close relative of DOT 5, but it is not. It is a glycol fluid like DOT 4. Under the US standard FMVSS 116, DOT 5 silicone is the only DOT fluid that must be dyed purple. That makes it easier to tell apart in the reservoir.
Because DOT 3, DOT 4 and DOT 5.1 share a glycol base, they can be mixed, although a full change to fresh fluid is better. DOT 5 cannot be mixed with any of them.
Why classic owners like silicone
It does not absorb water. Glycol fluids pull in moisture from the air over time. That water lowers the boiling point and can corrode cylinders and pipes from the inside. Silicone fluid is hydrophobic, so it does not take on water this way. This is why fluid makers and classic parts suppliers recommend it for historic cars and cars that spend most of their lives in storage.
It will not damage paint. Glycol fluid can damage painted surfaces if it is spilt. Silicone fluid makers state theirs will not harm paint, which matters on a show car or a fresh restoration.
It has a high boiling point. The minimum dry boiling point in the standard is 260°C for DOT 5, compared with 230°C for DOT 4.
Classic car needs some care?
Servicing, recommissioning and mechanical restoration for older cars across Kent.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
The downsides of DOT 5
Silicone is not perfect, and it is not right for every car.
- Pedal feel. Silicone fluid is more compressible than glycol fluid, which can give a slightly spongy pedal. If your pedal already feels soft, read our guide to a spongy brake pedal first.
- Bleeding takes care. DOT 5 absorbs a small amount of air, so the system needs careful bleeding.
- Water does not disappear. Because silicone does not absorb water, any moisture that does get in can separate out rather than mix in. Over time this can lead to water pooling in the system.
- ABS. DOT 5 is generally not considered compatible with anti-lock braking systems. Most older classics have no ABS, but some later ones do. If your car has ABS, follow the maker’s fluid specification.
Why you must never mix DOT 5 and DOT 4
This is the most important point in the whole article. Every silicone fluid maker we checked warns against mixing.
- Automec says DOT 5 and glycol fluids are chemically incompatible and can turn to jelly, leading to system failure.
- EBC warns that mixing can damage hydraulic seals and the whole brake system.
- Glycol and silicone mixtures can also trap moisture and corrode parts.
In practice, the danger comes from someone topping up the wrong fluid. A car converted years ago may have no obvious sign of it. Moss Motors sells a warning label for the reservoir for exactly this reason. If you have a converted car, label it and note it in the history file.
If you think fluids have been mixed, do not drive the car. Brakes that are not working properly can fail without much warning. Get it checked first.
Converting a classic to DOT 5: what is involved
Sources disagree on how far you need to go.
Some suppliers, such as MGBits and Moss Motors, say you can drain the glycol fluid thoroughly, then refill and bleed with silicone. MGBits also recommends making the change when you fit new master and slave cylinders or repair kits.
Automec takes a stricter line. It says the whole system should be dismantled, cleaned of old fluid with denatured alcohol or brake cleaner, dried with compressed air and rebuilt with fresh seals before filling.
Our view is that the stricter route is the safer one. Any glycol left in a cylinder or pipe can react with the silicone. The best time to convert is during a brake rebuild, when the cylinders are apart anyway. Remember the clutch: if it is hydraulic and shares the reservoir or fluid type, it needs the same treatment.
Converting back from DOT 5 to DOT 4 needs the same care in reverse.
Should you stay on DOT 4?
DOT 4 is a perfectly good choice for many classics, as long as it is changed regularly. Because it absorbs water, it needs replacing at intervals. How often depends on the car and the maker’s advice, so check your handbook. Our guide on how often to change brake fluid explains the signs and the reasons.
A fair rule of thumb:
- If the car is used regularly and serviced on time, DOT 4 changed on schedule works well.
- If it sits for long spells or you are rebuilding the brakes anyway, DOT 5 is worth considering.
- If it has ABS, follow the maker’s specification.
For more on drum brakes, wheel cylinders and pipes, see our guide to classic car brakes.
How we can help
We look after classic and vintage cars as part of our classic car servicing in Kent. We come to you in Aylesham, Canterbury, Dover, Deal, Sandwich, Folkestone and the villages in between. We can check which fluid your car is running, look at the condition of the brakes and talk you through whether a conversion makes sense.
Labour is £30-£50 per hour depending on the job and how difficult it is. Parts are extra. We agree the price with you on WhatsApp before any work starts.
Frequently asked questions
Is DOT 5.1 the same as DOT 5?
No. DOT 5.1 is glycol based, like DOT 4. DOT 5 is silicone. DOT 5.1 must not be mixed with DOT 5.
How can I tell if my classic already has silicone fluid?
DOT 5 is dyed purple under the US standard, so colour is a clue. A reservoir label or a note in the history file is more reliable. If in doubt, ask a mechanic to check before you top up.
Can I just top up with DOT 5 to start the switch?
No. Fluid makers warn that silicone and glycol fluids must not be mixed. The old fluid has to come out completely first.
Does silicone fluid ever need changing?
Some suppliers say a system filled only with silicone can last the life of the car. Water can still separate out over time, so it is worth having the brakes checked during servicing.
Need help? Send a WhatsApp to 07563 350335 with your car’s make and model and we’ll come to you.
Classic car needs some care?
Message us on WhatsApp with your reg and postcode for a price before any work starts.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
