Recommended Brake Fluid Replacement for Trailers
DOT 3 or DOT 4 is what should be used in your boat trailer. DOT 5 cannot be used in trailer brake systems unless specifically stated by the actuator and brake manufacturer. DOT 5 does not absorb water, and the silicone in the DOT 5 makes the seals swell and can bind up caliper pistons. Do not use DOT 5 or silicone brake fluids in any tie down engineering brake system including individual actuators, disc brakes, drum brakes or solenoids.
Many experts recommend changing the brake fluid every year or two for preventative maintenance. Their rationale is based on the fact that glycol-based brake fluid (DOT 3 & 4) starts to absorb moisture from the moment it is put in the system. The fluid attracts moisture through microscopic pores in seals and exposure to the air. The problem is obviously worse in wet climates where humidity is high and with marine trailers constantly subjected to dunking in lakes, rivers or oceans.