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Why Bleeding Hydraulic Brakes Matters
Air in trailer brake lines compresses under pressure and reduces clamp force at the calipers. On the road this shows up as longer stopping distances, delayed engagement from the actuator and uneven braking at the wheels. Bleeding removes trapped air and replaces aged fluid so the system responds promptly.
Tools, Fluid and Preparation
Use the correct brake fluid for your actuator and calipers, a clear bleed hose, a catch bottle and suitable spanners. Park on level ground, chock the wheels and fill the reservoir in the surge coupling or electric over hydraulic actuator to the marked level before starting.
Step-by-Step Bleeding Process
Attach a clear hose to the caliper bleed screw furthest from the fluid source and place the hose end in clean fluid inside the bottle. Generate pressure using the surge coupling or the brake controller’s manual override for EOH systems (for Deutsche HydroPro units this can be done from the app). While pressure is applied, open the bleed screw briefly to expel air and fluid, then close it before releasing pressure. Repeat until no bubbles appear, then progress to the next caliper. Keep the reservoir topped up throughout to avoid drawing in fresh air.
Common Mistakes to Avoid
Do not allow the reservoir to run dry. Avoid over-tightening bleed screws to prevent seat damage. If a caliper will not bleed cleanly, check for sticking pistons, perished hoses or loose unions that may admit air on release.
Service Frequency and Fluid Choice
Replace fluid at least annually, and more often for marine use. Always use the specified fluid type for your actuator and calipers. Mixing incompatible fluids can damage seals and reduce braking performance.
Parts and Components
If you find seized pistons, damaged hoses or leaking fittings, replace those parts before final bleeding. Our online store has hydraulic calipers, hoses and fittings and complete hydraulic brake kits to restore performance.
Bleeding Hydraulic Brakes – FAQs
Which wheel should I bleed first?
Start at the caliper furthest from the fluid source and work towards the nearest so air is purged from the longest lines first.
Can I pressure-bleed an electric over hydraulic system?
Yes. Follow the actuator manufacturer’s instructions and keep the reservoir topped up. Do not exceed the recommended bleed pressure to protect internal seals.
How do I know all the air is out?
Bleed hoses run clear with no bubbles, actuator operation returns to normal, the surge coupling stroke reduces to its usual range and wheel braking feels even on a low-speed stop test.