Can not get YZ450F front brakes to not be spongy

A buddy of mine's 250 is the same way. his is a 16 model

Rebuild the caliper and master cylinder and use new crush washers

I don't think so, I've ridden a friends YZ with steel braided line and an oversized kit and it still felt spongy. I had the best result replacing the rotor, pads and line with stock stuff and making sure you bleed it correctly.

Try bleeding the brake hose at master cylinder also, just pump lever, hold in & crack the hose bolt at cylinder, this worked for me on a bike I was having trouble building pressure..

Do you have braided line? That helped a lot on my 15 250f.

Hold brake lever in overnight.
Tie it to throttle with a bungie cord, turn handle bars left so lever is high.
Tap caliper, line and MC with screwdriver handle occasionally.

If that doesn't work, rebuild MC, caliper, and get a new line. All at once or in steps.

Brake fluid can absorb moisture in the air, be sure you’re using fresh unopened fluid.

Not sure if it would work on your bike but on my YZ250 I ended up putting a front brake off a CRF250 on it and it was so much better.

Take the caliber off the fork and hold it above the master cylinder and then bleed the system. Make sure you put something between the pads when you bleed, I put an old brake pad. After you bleed the system zip tie the lever closed over night and you should be good to go.

Cycle trader uses/used kx master cylinders on there bikes. I’ve also heard of people running older yz calipers as they have a larger piston.

I did this^^^

Have a buddy with a good connection at jgr and picked up a couple of their last Yamahas they had and their bikes all had the older caliper/master cylinder conversion. I had the same problem with the front brakes, new oversized rotor, braided line and pads and still couldn’t get the spongyness out of them. Swapped the brakes off the jgr bike and it’s insane the difference. I have one more set up I am going to sell if anyone is interested.

Which years had those calipers with bigger pistons? I can't stand my spongy front brake on my 19 YZF 450.

Had the exact same problem on my 15' yz250. Tried everything. Steel braided line, RBF 600 fluid, Galfer oversized rotor. Nothing helped. Finally bought a used 2017 CRF450R complete front brake system on ebay for a $100. Bolts right up. Just use the YZ bracket. Night and day difference. Feels like the Brembos on my KTM now.

After trying everything mentioned above , the solution was a used brembo master off ebay for $65.
Amazing results.

This soft/spongey feel is generally due to the master cylinder piston size. YZ is often commented from pros as having that spongey feel. I don't remember what the YZ master cylinder piston size is. But 8mm, 9mm, 10mm are all going to have various levels of firmness and maximum pressure strength. "Generally" a larger piston will yield a "harder" feel but can sometimes not provide the same level of max brake pressure vs a smaller piston with a similar master cylinder. Ofcourse, this feeling of sponge can be exacerbated by multiple things within the brake system. Old brake fluid, bubbles in the system, old brake lines, bent rotors, bent brake caliper hangers, this would cause the pads not to sit flush with rotor etc etc. In the VERY rare case the for lug could be bent causing the front brake caliper tomnot sit in line with the rotor. But like I said, if all those things are okay then you should try a different master cylinder. 10mm Brembo from KTM (older ktms... 2004-7 ish I think, or a CRF master. Those will provide a firmer feel than the YZ master cylinders almost immediately if you bleed them correctly after installing. Bleeding is not rocket science, there are some tricks that make the process easier. Message me if you need those tips. Good luck!