How to Bleed and Replace Brake Fluid on a 1991 Polaris Trail Boss 350
Shop parts for a 1991 Polaris Trail Boss 350 ATV.
This stepbystep guide from MotoSport walks you through safely flushing old brake fluid and bleeding the hydraulic system on a 1991 Polaris Trail Boss 350. Follow every step carefully and use DOT 3 brake fluid (or OEMapproved equivalent) only. If you see dark, milky, or contaminated fluid, replace it immediately.
Tools & supplies
- DOT 3 brake fluid (new, unopened)
- Clear 3/16" or 1/4" plastic bleed hose
- Catch bottle or jar
- Wrench set (bleeder nipple size usually 8mm or 10mm)
- Socket set and jack/stands to lift the ATV
- Rubber gloves, safety glasses, shop rags
- Turkey baster or fluid syringe (optional for reservoir predrain)
- Brake cleaner and small brush
Safety first
- Work on a level surface; block wheels to prevent movement.
- Wear eye protection and gloves; brake fluid damages paint and plastic.
- Do not let dirt enter the master cylinder reservoir.
Overview of the Trail Boss 350 braking components
The 1991 Trail Boss 350 uses a simple hydraulic brake system with a single master cylinder and hydraulic lines feeding the front calipers and rear brake assembly. On older machines like this, rubber hoses and seals can be brittle, so inspect lines, banjo fittings, and bleeder screws for corrosion or leaks while you work.
Preparation
- Park ATV on level ground, engage parking brake, and remove the key.
- Clean around the master cylinder reservoir cap to prevent contamination.
- Remove the reservoir cap and diaphragm; if fluid is very dark, use a turkey baster to remove most old fluid from the reservoir before starting.
- Lift the ATV if needed so you can access calipers and rear brake easily; support securely on stands.
- Locate bleeder nipples on the front calipers and rear brake housing. Have your clear hose and catch bottle ready.
Bleeding procedure (recommended: gravity first, then pressure or pump method)
Use either the pumpandhold method (helper pumps lever/pedal) or gravity/oneman method with a hand pump/bleeder kit. Sequence by bleeding the farthest caliper from the master cylinder first, then the closest.
- Top off the reservoir with fresh DOT 3 fluid and keep the cap loosely in place to limit contamination.
- Attach clear hose to the bleeder nipple and place the other end in the catch bottle partially filled with fluid to prevent air reentry.
- With a wrench on the bleeder, open the nipple about a quarter turn.
- If using a helper: have them firmly pump the brake lever/pedal several times and hold it down. Open the bleeder to let fluid and air escape. Close the bleeder before they release the lever. Repeat until clear fluid with no bubbles appears, then tighten the bleeder securely.
- If oneman method: use a squeezetype brake bleeder or hand pump to pull fluid while opening/closing the nipple as directed by your tool's instructions. Maintain reservoir level between steps to avoid drawing air into the system.
- Repeat for each wheel in order from farthest to nearest (typically: rear, then front right, then front left, or follow the actual line routing on your unit). Always monitor reservoir fluid and never let it run dry.
- Once all wheels show clear fluid and no bubbles, tighten all bleeder nipples to proper snugness and wipe clean. Reinstall the reservoir diaphragm and cap.
- Top off fluid to the proper level, secure cap, and clean any spilled fluid from painted surfaces with water immediately.
- Carefully test the brake lever/pedal feel with the ATV still supported. Brakes should feel firm. If spongy, repeat bleeding until firm.
Postservice checks
- Inspect lines, banjo bolts, and crush washers for leaks after a short test ride. Replace crushed or leaking copper washers and any cracked hoses.
- Dispose of old brake fluid according to local hazardous waste rules.
- If the master cylinder is old and the system remains soft after bleeding, consider a master cylinder rebuild or replacement. Internal seals on a 1991 Trail Boss can fail with age.
Modelspecific notes for the 1991 Polaris Trail Boss 350
- Stock components are aged; check rubber hoses and the master cylinder for cracking and seepage. Replace brittle lines proactively.
- Banjo bolts and bleeder screws on older Polaris units often seize from corrosion. Penetrating oil and careful heat may be needed; replace any rounded or damaged fittings.
- OEM service parts such as seals, washers, and bleeder screws are widely available through MotoSport if you need rebuild kits or replacement hoses.
- Rear brake hardware on this model can be a mix of drum or hubmounted shoes depending on aftermarket changesidentify your exact setup before ordering parts.
- Use DOT 3 fluid unless the vehicle has been converted with components rated for DOT 4; mixing types is not recommended unless compatible by specification.
Troubleshooting
- Persistent spongy lever after repeated bleeding: suspect internal master cylinder seal failure or an undiscovered air leakinspect and rebuild/replace as needed.
- Brake pedal that sinks slowly under pressure: likely master cylinder seal leak or line collapse; remove and benchtest the master cylinder.
- If bleeder nipples won't open or are rounded, replace them and the mating hardware to ensure a reliable seal.
Keeping brake fluid fresh and the system free of air is essential for safe stopping performance. For parts, rebuild kits, hoses, and brake hardware specific to the 1991 Polaris Trail Boss 350, visit MotoSport's parts page linked above to find OEMfit components tailored to your MMY.
Related Shopping Categories
Shop Brake Fluid for a 1991 Polaris Trail Boss 350 ATV.
Shop Brake Bleeders for a 1991 Polaris Trail Boss 350 ATV.
Shop Brake Pads for a 1991 Polaris Trail Boss 350 ATV.
Shop Brake Lines for a 1991 Polaris Trail Boss 350 ATV.
Shop Brake Tools for a 1991 Polaris Trail Boss 350 ATV.
Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.