Mineral oil, rapeseed oil, castor oil, water-glycol and rendered tallow can all carry hydraulic pressure, but the fluid and the seal material must be chosen as a pair, because the wrong pairing swells or shrinks the seal and the system leaks. Soak a trial piece of the actual seal in the actual fluid for a day and measure the change before building around the pair, and never mix brake fluid with oil in one system.
Part of Hydraulic Systems
Hydraulic fluids differ in lubrication, temperature range, fire risk and what they do to rubber. This article compares the options a village can actually make or salvage.
The Options Side by Side
The carrier’s table, with the points that matter for seals:
| Fluid | Pros | Cons | Seal and fire notes |
|---|---|---|---|
| Mineral oil | Best lubrication, wide temperature range | Flammable, petroleum-dependent | Swells natural rubber; leather cups work |
| Vegetable oil (rapeseed) | Renewable, good lubrication | Oxidizes and thickens over time | Test rubber seals; leather works |
| Castor oil | Excellent lubrication, high viscosity | Expensive, limited supply | Test seals |
| Water-glycol | Fire resistant, cheap | Poor lubrication; the carrier says it corrodes steel | Test seals; glycol is toxic to swallow |
| Tallow (rendered fat) | Always available | Poor in cold weather; goes rancid | Leather works |
The carrier’s best practical choice is filtered rapeseed or other vegetable oil, changed every 6-12 months. The sources are in Oil Pressing at Scale and Animal Fats (Tallow, Lard).
The Fluid and the Seal Are a Pair
The corpus warns that natural rubber swells in oils (Hoses and Tubing). The salvage chapter lists brake fluid and transmission fluid as hydraulic fluids (Fluids: Oil, Coolant, Brake Fluid). The two are different chemistries: brake fluid is not mineral oil. A master cylinder or caliper made for brake fluid has rubber chosen for it, and putting mineral oil in it can swell the seals, and putting brake fluid into a system with seals made for oil can soften or harden them. Which rubber survives which fluid is recalled general knowledge and not established in the corpus, so test rather than assume.
Method: the soak test.
- Measure a spare seal (or a cut piece of the same rubber) across its section with calipers or a gauge.
- Soak it in the intended fluid at the working temperature for a day.
- Measure again and feel it. More than a slight change in size, or any softening, stickiness, cracking or hardening, means reject the pair.
Leather cup seals soaked in oil are the corpus’s standard answer for oil systems (Gaskets and Seals).
Fire and Toxicity
- Mineral and vegetable oils burn, and a fine spray from a burst line burns easily near a forge. Keep sand or an extinguisher nearby and shield lines near heat.
- Water-glycol resists fire but the glycol is poisonous if swallowed. Label the container and keep it from children and animals. The corpus gives no dose, so do not work with it near drinking vessels.
- Fluid that has been in service is dirty; do not pour it into water.
Common Mistakes
- Topping off with whatever is to hand. A mix of vegetable oil and mineral oil, or any oil in a glycol system, can foam, separate or attack the seals.
- Salvaging a cylinder and filling it with the wrong fluid. Check the seals first.
- Forgetting that vegetable oil oxidises. Gummy deposits in a valve are the sign; change it.
Safety
Every fluid here injects through skin at pressure
Mineral oil, vegetable oil, water-glycol and tallow all pass through skin as a fine pinhole jet from a pressurised line, and the injury is a surgical emergency that can cost the limb. Never search for a leak with a hand; pass card or wood across the fitting and look for fluid on it, at low pressure and with everyone clear of the jet. Anyone who may have been hit needs urgent surgical care at once; do not wait for pain or swelling.
What’s Next
- Learn next: Hydraulic Fluids — keeping the chosen fluid clean
- Combine with: Seal Systems — the seals the fluid touches