The topic of using silicone versus regular brake fluid comes up several times a year among WWII jeep collectors.

Regular brake fluid can be a good paint remover. Of course, if you plan ahead for a spill, it’s no big deal. But is silicone brake fluid the ultimate cure for poor paint surfaces and poor maintenance technique? Please note that while silicone will not remove paint, it can cause what is called “cupping” where the paint will not adhere to the surface being painted. So it could be a concern if you have to do any touch ups at a later time.

How to determine if it has silicone liquid or glycol? To determine if an unknown sample is glycol or silicone fluid, place some in a glass jar. Add water one drop at a time. The glycol fluid will dissolve in water with a slight milkiness, but the silicone fluid will form two distinct layers, with water on the bottom. Silicone stays clean and clear in a brake system, but old glycol looks rusty after years of use. The purple tint transfers from the silicone to the glycol fluid if you mix them.

This guy has a pretty good site and seems pretty balanced on the glycol vs. silicone talk

Which one to use? It seems to be decided by a few factors:

1. If your preventative maintenance skills are lax, then using silicone is probably your best option. If you can change your brake fluid every couple of years, maybe glycol is best for you.

2. If you can’t fill the master cylinder without spilling it on the painted surface, maybe silicone is the way to go. Keep in mind, however, that while silicone doesn’t eat away at paint, it prevents it from adhering to the surface and causes “cupping.”

3. If you have more money than time, maybe silicone is the best for you. In most places, the same amount of silicone is several times the cost of glycol. But on the other hand, you will need to replace glycol several times more than silicone.

4. Do you drive your jeep through water, wet roads when it rains? Jeeps, like many brake systems, are open to the atmosphere. The glycol is designed to absorb moisture and has inhibitors designed to reduce corrosion. Silicone is not designed to absorb water. Moisture will collect in the silicone and can lead to corrosion.

5. Spongy feeling. Silicone has a higher compressibility than glycol, which can cause a spongy feeling on the brake pedal. Silicone has a slower rate (higher viscosity) than glycol. This may explain why it can be difficult to bleed air from silicone-equipped brakes. Once the air mixes with the silicone (bubbles), it can take a long time to purge it.