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I cant get my rear brake on my 2014 yz250f  to pressure up. I have a brand new caliper btw. I used dot 3 and 4 fluid would that have caused something? Any help is appreciated! 

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Fluid is fine. You either have an air bubble in the system somewhere, or a bad master. I see the part about the "brand new caliper". Did the problem start after you replaced the caliper? Or did it have the problem before, causing you to replace the caliper in an attempt to fix it?

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Most Japanese cars and dirt bikes, have a slow fill style master cylinder, and they can be difficult to get the air to move towards the bleeder.

Get you a large plastic syringe and fill it with brake fluid.  Put a piece of rubber hose on it, and attach the other end the the bleeder.  Open the bleeder about a turn.  Remove the lid off of the master cylinder.  Pump the brake fluid through the caliper and back up to the master cylinder.  It's messy, but effective if you can't get it to bleed by the common method.

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Most Japanese cars and dirt bikes, have a slow fill style master cylinder, and they can be difficult to get the air to move towards the bleeder.
Get you a large plastic syringe and fill it with brake fluid.  Put a piece of rubber hose on it, and attach the other end the the bleeder.  Open the bleeder about a turn.  Remove the lid off of the master cylinder.  Pump the brake fluid through the caliper and back up to the master cylinder.  It's messy, but effective if you can't get it to bleed by the common method.

Okay

I tried it but i didnt use a tube but i will try with a tube.

Thank you

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I think that DOT 3, 4, and 5.1 are somewhat compatible because they are all glycol based.  DOT 5 is not compatible with the others because it is silicone based.

Personally, I wouldn't mix DOT ratings and would flush the old type before adding the new type.  Glycol-based fluids absorb water (even vapour from the air) and become degraded.  This is also a good reason to completely replace the old type rather than just adding a new type to it.

The reverse bleeding approach suggested by W.W. above is usually what I use also.  I bought a brake bleeding kit from Harbor Freight Tools but I never use it because the syringe and tube work so well.

Edited by Max17
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Brake fluid is corrosive to painted surfaces, and maybe some others as well.

It's water soluble, so rinse it off with water.

That is also why you should change brake fluid on all cars every 2 years regardless of mileage, it's hygroscopic and absorbs moisture, and moisture isn't good for the brake system.

On my bikes, I change it every few months.

A bottle of brake fluid is cheap, so buy a new bottle each time you change or bleed a brake or clutch system. 

Toss that bottle that has been setting on the work bench next to the dried up Carnuba wax lol.

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Okay so the brake is very weak. I push the lever all the way down and it stops the tire barely. I ca force the tire o spin when the brake is fully engaged.

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You cannot compress liquid, so if the brake pedal is spongy and has excessive travel, you either still have air, or maybe a caliper slide pin is stuck and the caliper is flexing.

What was the original reason for replacing the caliper?

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10 hours ago, Tboy298 said:

The threads for thebrake line bolt were stripped.

Were they stripped from over tightening, or did the caliper take a hit from an incident?

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Over tightening i believe. I bought the bike used so i didnt know the threads were like that but i believe its the slide pin you were talking about. ill have to take a look at it when i get off work

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With the brake pads removed, the caliper should slide easily and fully, back and forth on the slide pins. 

If not, they are likely dry and chalky, or bent.

Make sure the are clean and well greased with synthetic caliper slide pin grease.

 

 

Edited by WALKINGWOUNDED
added info

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