How to Bleed and Replace Brake Fluid on a 1990 Yamaha Bigbear 350 4X4

Shop parts for a 1990 Yamaha Bigbear 350 4X4 ATV.

This MotoSport guide walks you through a complete, safe brake-fluid replacement and bleed for a 1990 Yamaha Bigbear 350 4X4 (MMY). Follow these steps carefully to restore firm, consistent braking and inspect the braking system for age-related wear common to this model.

What you need

  • Manufacturer-specified brake fluid (see reservoir cap marking & owner’s manual; DOT 3 or DOT 4 may be used only if specified)
  • Clear tubing to fit bleeder nipples, clean catch bottle, gloves, safety glasses
  • Wrench for bleeder nipples, screwdriver, shop rags, funnel or syringe
  • Brake-fluid syringe, hand or vacuum bleeder (recommended), torque wrench if removing components
  • Replacement rubber hoses or caliper seals if lines or boots are cracked

Model notes — what owners of the 1990 Bigbear 350 should know

  • The Bigbear 350 uses a simple hydraulic system with a remote reservoir. Reservoir caps are typically marked with the proper DOT rating — confirm before filling.
  • Aging rubber hoses, banjo bolts, caliper seals, and master-cylinder diaphragms are common failure points on older Bigbears. If fluid is dark or milky, expect internal seal deterioration or water contamination.
  • The brake assembly is compact on the Bigbear; protect plastic fenders and bodywork from spilled fluid when working.
  • The Bigbear does not have ABS. That simplifies bleeding but means contamination or air will directly affect pedal/lever feel.
  • If the master cylinder is leaking or the lever/pedal travel does not firm up after bleeding, plan to rebuild or replace the master cylinder and caliper seals with OEM-spec parts.

Safety and prep

  • Work on a level surface with the engine off, key removed, and parking brake engaged. Block wheels for stability.
  • Wear safety glasses & gloves. Brake fluid damages paint & plastics — cover bodywork with rags.
  • Clean the reservoir cap area before opening to avoid contamination.
  • Use only clean, unopened brake fluid from an undamaged container.

Step-by-step: drain, replace, and bleed

1. Inspect and prepare

  • Inspect reservoir fluid color. Dark or dirty fluid means a full replacement is required.
  • Top up reservoir to the max line with fresh fluid so you don’t run dry during bleeding.
  • Identify bleed nipples on front calipers and rear drum or caliper assemblies. Clean them before attaching tubing.

2. Drain old fluid from reservoir

  • Use a syringe or turkey baster to remove as much old fluid from the reservoir as possible. Dispose of fluid legally.
  • Refill reservoir with fresh fluid to the max line to start the bleeding process with clean fluid in the system.

3. Recommended bleed order

  • Use the furthest caliper from the master cylinder first, then work closer. On the Bigbear 350 this usually means: rear (if independent) – then front right – then front left. If you have a single rear drum, start at the front opposite the master cylinder side if uncertain.

4. Bleeding methods (choose one)

Vacuum or pressure bleeder (preferred)
  • Attach vacuum pump to the bleeder nipple with clear tubing; open bleeder nut slightly. Pump until clean fluid and no air bubbles appear. Keep reservoir topped up.
  • If using a pressure bleeder at the reservoir, pressurize per tool instructions & open each bleeder in sequence until clean, bubble-free fluid flows.
Two-person pump-and-hold
  • Person A slowly pumps the brake lever/pedal several times and holds it compressed.
  • Person B opens the bleeder nipple briefly to allow fluid/air to escape, then closes it before A releases the lever/pedal. Repeat until fluid is clear and no bubbles appear.
One-person syringe or gravity bleeding
  • Attach tubing from bleeder to a catch bottle, lower bottle to encourage flow. Carefully open bleeder to let gravity push fluid through — this is slow and requires patience; keep reservoir topped up.

5. Bleeder technique details

  • Open the bleeder only enough to allow fluid to escape; don’t strip threads. Close the bleeder before releasing the lever/pedal if using manual pump-and-hold.
  • Watch the reservoir level & top up frequently to avoid introducing air into the master cylinder.
  • Replace the bleeder cap when finished on each caliper. Wipe any spills immediately.

6. Bench-bleed the master cylinder if removed or if air persists

  • If you remove the master cylinder or it contains trapped air, bench-bleed it before reinstalling: secure it in a vise, fill the reservoir, use short hoses from the outlets back into the reservoir, and cycle the piston until bubbles stop.

7. Final checks

  • Confirm firm lever/pedal feel at rest and under operation. If travel remains spongy, repeat bleeding until all air is cleared.
  • Check for leaks at banjo bolts, bleeder nipples, hoses, and caliper seals after a short test ride and re-torque fittings if required.
  • Dispose of old brake fluid according to local regulations.

Troubleshooting

  • Persistent sponginess: suspect internal master-cylinder or caliper seal failure — rebuild or replace.
  • Dark fluid returns quickly: system contamination or internal material breakdown — inspect hoses, seals, and master cylinder.
  • Fluid level dropping: active leak. Don’t ride until repaired.

Maintenance tips

  • Replace brake fluid every 1-2 years or sooner if it darkens; moisture absorption reduces boiling point and promotes corrosion.
  • Keep a small supply of manufacturer-specified fluid and a bleed kit in your garage for quick maintenance.
  • On older Bigbears, proactively inspect rubber hoses, caliper boot seals, and banjo washers for wear to avoid roadside failures.

Follow these steps and maintain clean fluid to keep your 1990 Yamaha Bigbear 350 4X4 braking reliably. If you need replacement parts or OEM-style seals, use the MotoSport parts page above to find the correct components for your MMY.

Related Shopping Categories

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Shop Brake Lines for a 1990 Yamaha Bigbear 350 4X4 ATV.

Shop Brake Tools for a 1990 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.