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I see many posts where owners and mechanics alike complain about encountering difficulty bleeding brakes on ATV's where the calipers have been replaced or rebuilt. The problem always the same, "I can't pump all the air out of the system" or "I used a vacuum bleed tool and cant get all the air out of the system".

I used to have the same problem and I gave up on the vacuum bleeder, although it was better than trying to pump air out of the system with the master cylinder.

Here's the problem: the master cylinder only has about a 10cc capacity. With each pump of the lever, only about 5cc of fluid is displaced to the calipers. The calipers have about 1cc capacity when the piston is fully depressed into the caliper body and most Polaris models have at least 3 calipers. The lines have about 10cc of capacity so there is at least 13cc of air that needs to be displaced with a 5cc injection of liquid. Now since air tends to rise to the highest point in a liquid, each time you pump the lever you inject appx.5cc of fluid if the master cylinder has been bled ahead of system bleeding. If you are patient, you can eventually pump all the air out, but it will take several hours with pauses to allow tiny air bubbles to collect into a large bubble ready for expulsion.

Now I endeavor to bleed a system in as little time as possible, so here's the method I use when a pressure bleeder is not available: I first fill the MC reservoir with new fluid from a sealed container. Knowing that air tries to rise to the highest point in the system, I turn the handlebar to the right (for a MC on the left end of the bar) to position it as high as possible. Sometimes I will put a jack under the LH front a-arm and raise the left front wheel to raise the MC to a higher point.

First step to bleeding the system: I pump brake fluid from an oil can into the line where it attaches to the MC. This is messy and it accomplishes little other than displacing some of the air in the lines with fluid. The bleeder screws may be open during this and closed as soon as fluid reaches the screw.

Second step: Place a finger tip lightly over the brake line port on the MC and pump the cylinder - air should be forced past the finger tip and upon release of the lever, suction should be felt and fluid should be drawn from the master cylinder reservoir into the cylinder. I again pump the cylinder slowly to expel air and fill the chamber with fluid when the lever is released. When I get more fluid than air, I keep the MC piston depressed and connect the line.

Third step: Go pee, get an iced tea, play a game on the tablet, read a service manual or comic book, work on something else for awhile or go to bed and return to work on it at a later time (do not drink beer or smoke weed if you intend to continue to work on it).

Fourth step: Knowing the air will rise in the system, I gently pump the brake lever just to move the MC piston about 1/4 inch with each operation and then release it. With each operation a small amount of fluid is pumped into the line and the air it displaces is released into the MC reservoir. Continue this until no or very little air is being released. At this point you should be able to bleed the brakes conventionally.

NOTE: it is best to keep the fluid suction/pressure relief hole in the cylinder covered with fluid, but if the level drops and air is sucked in, it just extends the process by a minute or so.

NOTE: conventional bleeding consists of pumping the brakes until pressure is felt, then while holding pressure on the system, crack the bleed screw on each caliper to let air out and close the bleed screw before releasing the brake lever. Re-pressurize the system and repeat for each caliper and each line fitting finishing at the master cylinder line fitting.

If the brakes are not satisfactory after setting unused for 12 hours or more, turn the hanlebar to position the MC at it's highest point and work the lever gently for several pumps to remove the spongy feeling. Maybe you will need to pressurize the system and crack the line fitting at the MC one last time.

If the brakes are not satisfactory after this procedure, go to step 5.

Step five: Remove one caliper at a time and remove the brake pads. Pump the caliper piston 3/4 out of the caliper body. Holding the caliper so the line fitting is at the apex, use a c-clamp and press the piston back into the caliper. This will force fluid and any residual air from the caliper back to the MC. Repeat for each caliper.

After all this if the brakes are still unsatisfactory, troubleshoot the system and replace the defective component.
 

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i would like to add to latebird brake bleed. all good advice. the problem is air is trapped in the banjo line fitting.
i drill another small hole in the banjo bolt so when the banjo fitting is in the flat horizontal postion, the banjo
bolt is vertical with the bolt head down. in this position the new hole in the banjo bolt is the at the top of this cavity.
so after all the standard air bleeding , i unbolt the master cyl from the handle bars and hold an old hair clipper
angainst the banjo bolt and the vibration bleeds the remaining air out of the fitting.. works for me.
 
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Discussion Starter · #5 ·
i would like to add to latebird brake bleed. all good advice. the problem is air is trapped in the banjo line fitting.
i drill another small hole in the banjo bolt so when the banjo fitting is in the flat horizontal postion, the banjo
bolt is vertical with the bolt head down. in this position the new hole in the banjo bolt is the at the top of this cavity.
so after all the standard air bleeding , i unbolt the master cyl from the handle bars and hold an old hair clipper
angainst the banjo bolt and the vibration bleeds the remaining air out of the fitting.. works for me.
Yes - what ever you can do to get the air out improves the feel of the brakes and many times I remove the master cylinder to get trapped air out, but it;s not always necessary. Every job is different and can present extraordinary difficulty requiring extra effort and diligence in expunging the air from the system.
 
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Well @latebird, I want to report that I had major success with this.

So much that on my last round of step 5 I blew the piston out and had to start over haha...

Pro tip, dont let the piston push out, and grease/clean your slide pins. :poop:
 
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