Polaris Sportsman 700 Rear Brake Master Cylinder: Diagnosis and Repair
Polaris Sportsman 700 Rear Brake Master Cylinder Failure
When your Sportsman 700 EFI loses rear brakes, it’s almost always the master cylinder. The pedal goes soft, or stops responding entirely. It happens fast once it starts.
What Actually Fails
Inside the master cylinder is a rubber seal that holds pressure behind the piston. After a few hundred to a couple thousand hours, that seal wears out. Brake fluid leaks past it. You press the pedal and nothing happens—there’s nowhere for the fluid to go.
Sometimes it’s not the seal. Mud gets past the outer dust cover and clogs the internal passages. The cylinder still works mechanically, but can’t build pressure.
When Flushing Works
If the problem is accumulated gunk, flushing helps. Disassemble the cylinder, soak the parts in brake cleaner to dissolve the buildup, use fresh brake fluid to lubricate the seals, and reassemble. Many owners report this fixes a soft pedal that crept on gradually.
The catch: this only works if the seal is still intact. If it’s torn or permanently deformed, flushing won’t fix it. You’ll bleed the brakes perfectly and the pedal will still be soft, or fail again in a few miles.
When You Need Replacement
Polaris doesn’t sell rebuild kits for the Sportsman 700 rear master cylinder. You can’t buy individual seals. Your option is to replace the whole unit. OEM replacements are available through Polaris parts dealers and online retailers, and aftermarket copies are also made but tend to be less reliable.
Some riders try to disassemble and rebuild with seals from a general motorcycle supplier. This is a gamble. It usually doesn’t work perfectly.
How to Tell the Difference
Bleed the brakes first. Have an assistant pump and hold the rear brake pedal while you open the bleeder screw on the caliper. Brake fluid should flow out steadily. Repeat this cycle eight to twelve times until no air bubbles come out. Keep the reservoir topped up—never let it run dry or you’ll trap air in the system and have to start over.
Test the pedal. After a full bleed, it should feel firm. If it’s still soft, air is still trapped somewhere.
Park and wait overnight. If the pedal was firm after bleeding but soft the next day, fluid leaked out overnight. The seal failed. If it stayed firm, the problem was air and you’re done.
Bleeding Procedure
You need DOT 4 brake fluid, a clear tube that fits the caliper bleed screw, a wrench or hex key, and an assistant. Process:
- Have your assistant pump the brake pedal slowly three or four times, then hold it down hard
- Open the caliper’s bleed screw with your wrench. Fluid and bubbles will flow out
- When flow slows to a drip, close the screw and tell your assistant to release the pedal
- Top up the master cylinder reservoir
- Repeat until clear fluid with no air bubbles comes out
This usually takes 8–12 cycles. The critical part: don’t let the reservoir run dry. Air in the master cylinder means starting from scratch.
If Bleeding Doesn’t Fix It
Once you’ve bled properly and the pedal is still soft, the master cylinder seal is likely dead. Check the fluid level first—a low level means a leak somewhere in the line or caliper. If level is normal and the pedal is bad, the master cylinder failed.
Some riders remove the cylinder and try to clean and reseal it themselves. This rarely works if the seal was the main problem, but it’s free to attempt before buying a replacement.
Long-term Prevention
The rear brakes on a 700 don’t get used much, so they get forgotten. Check fluid level every season. Every two years, do a full fluid flush and bleed on both front and rear. If you ride in mud or water, inspect the rubber dust cover on the master cylinder. If it’s cracked or missing, mud is getting in and failure will happen sooner.