How to bleed and replace brake fluid on a 1991 Yamaha Bigbear 350 4X4
Shop parts for a 1991 Yamaha Bigbear 350 4X4 ATV.
Overview
This MotoSport guide shows a safe, repeatable process to flush and bleed the hydraulic brakes on a 1991 Yamaha Bigbear 350 4X4. Work carefully, keep contaminants away from the system, and use the correct brake fluid. This procedure is written for a stock hydraulic brake system on the Bigbear; if your machine has aftermarket calipers, master cylinder, or integrated parking-brake modifications, adapt the steps accordingly.
What you need
- Manufacturer-recommended brake fluid (use fresh, unopened container; see owner’s manual or parts listing if unsure)
- Clear plastic tubing that fits snugly on bleed nipples
- Brake fluid syringe or turkey baster (for reservoir removal)
- Catch bottle for old fluid
- Wrench or socket for bleed nipples (usually 8mm or 10mm)
- Clean rags and isopropyl alcohol for cleanup
- Latex or nitrile gloves and eye protection
- Assistant (recommended) to operate the brake lever/pedal while you crack bleed screws
Model notes for the 1991 Yamaha Bigbear 350 4X4
- Brake reservoir location: the front master cylinder reservoir is mounted on or near the handlebar master assembly; the rear master may be attached near the foot brake pedal. Confirm which reservoir feeds which circuit before starting.
- Brake type: the Bigbear typically uses simple hydraulic calipers/wheel cylinders without ABS or complex valves. That simplifies bleeding but makes keeping the reservoir topped critical to avoid introducing air.
- Fluid spec: Yamaha historically specifies DOT-rated mineral-based or glycol fluids depending on year and model. Use the fluid type recommended for your Bigbear and never mix DOT fluid types unless the manual allows it. If you don't have the manual on hand, obtain the correct part from MotoSport parts for your MMY.
- Age-related concerns: seals, hoses, and bleed screw threads on a 1991 unit can be brittle or corroded. Inspect rubber hoses and caliper boots; replace any cracked or bulging parts before bleeding to avoid leaks or seal damage.
Safety & preparation
- Park on a level surface, secure the ATV with the parking brake engaged, and block wheels if necessary.
- Wear gloves & eye protection. Brake fluid damages paint and plastic; cover bodywork and clean spills immediately with alcohol.
- Remove any luggage racks or parts that obstruct access to calipers, master cylinders, or bleed screws.
Step-by-step brake fluid replacement & bleeding
1. Identify circuits and plan sequence
Decide which reservoir controls which wheel(s). Typical bleed order is: wheel farthest from the master cylinder first, then progressively closer. For many Bigbear setups that means right rear, left rear, right front, left front. Adjust if your plumbing differs.
2. Top off and remove old fluid from reservoir
Clean the reservoir cap area to avoid contamination. Remove the cap and diaphragm, and use a syringe or turkey baster to remove as much old fluid as possible from the reservoir. Refill with fresh fluid to the fill line and replace the cap loosely; keep the reservoir topped during the entire procedure.
3. Prepare bleed ports and tubing
Place a clear tube over the bleed nipple on the caliper/wheel cylinder and route the tube into a catch bottle with some fresh fluid in the bottom to prevent air from being drawn back in. Have your assistant sit on the seat or press the brake pedal as required.
4. Manual (two-person) bleeding method
- Have the assistant pump the brake lever or pedal several times and hold firm pressure.
- Open the bleed nipple about a quarter turn to allow fluid and air to escape into the tube. Close the nipple before the assistant releases pressure.
- Repeat: pump, hold, open, close. Each cycle should yield fluid and, at first, air bubbles. Keep checking the reservoir level and refill as needed so air is not drawn into the master cylinder.
- Continue until all bubbles stop and you see only clear fluid. Then move to the next wheel following the planned sequence.
5. One-person vacuum or syringe method (alternative)
Attach a vacuum pump or syringe to the bleed nipple. With the master cylinder capped and topped, operate the vacuum to draw fluid through the system while occasionally having the lever/pedal cycled from released to pulled to move trapped air toward the bleed point. Keep reservoir topped.
6. Final reservoir flush
After bleeding all wheels, perform a final reservoir flush: remove a portion of fluid from the reservoir and refill with fresh fluid, then do one more short bleed at the closest wheel(s) to purge any remaining fluid in the lines. This ensures most old fluid is removed.
7. Tighten, clean, and test
- Torque the bleed nipples snugly (do not over-torque rusty nipples; replace if threads are damaged).
- Wipe off all spilled fluid and clean contact areas with isopropyl alcohol. Replace reservoir diaphragm and cap securely.
- Operate the brake lever/pedal to confirm firm, consistent pressure and travel. With the ATV secured, perform a slow test ride to confirm braking performance before returning to full use.
Tips & troubleshooting
- If a caliper keeps spitting air after repeated bleeding, the caliper piston seal may be leaking air in or the master cylinder may have internal leaks; inspect and rebuild or replace as needed.
- Sticky or soft pedal after bleeding can indicate trapped air or a failing master cylinder. Re-bleed, inspect components, and replace worn parts.
- Replace old rubber hoses if you see cracking, soft spots, or fluid swelling. On a 1991 Bigbear, aged hoses are common and can cause spongy braking or leaks.
- Dispose of used brake fluid responsibly per local regulations; do not pour it on the ground or into drains.
Common parts to inspect or replace on a 1991 Bigbear during a brake service
- Brake pads/shoes and rotors/drums
- Caliper slide pins, seals, and boots
- Master cylinder seals and reservoir cap diaphragm
- Brake hoses and bleed screws
Final checks
After service, double-check for leaks, confirm proper free play at the lever and pedal, and ensure the parking brake (if equipped) functions. Perform a low-speed braking test in a safe area to validate stopping distance and pedal firmness before riding aggressively or in remote areas.
For parts specific to your MMY 1991 Yamaha Bigbear 350 4X4, visit the MotoSport parts listing above to find the right master cylinder components, hoses, seals, and recommended fluids.
Related Shopping Categories
Shop Brake Fluid for a 1991 Yamaha Bigbear 350 4X4 ATV.
Shop Brake Bleeders for a 1991 Yamaha Bigbear 350 4X4 ATV.
Shop Brake Pads for a 1991 Yamaha Bigbear 350 4X4 ATV.
Shop Brake Lines for a 1991 Yamaha Bigbear 350 4X4 ATV.
Shop Brake Tools for a 1991 Yamaha Bigbear 350 4X4 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.
