If you switched to 3/4 ton axles, you have larger wheel cylinders (and calipers if it's disc brakes). The 1/2 ton master cylinder is a smaller bore than the 3/4 master cylinder, so it will travel further to displace the amount of brake fluid necessary to drive the hydraulics out, which means a longer pedal travel.
To fix it:
1. First, make sure that the brakes are adjusted properly. This is another reason for long pedal travel.
2. Then, verify the wheel cylinder sizes. I'm probably right, but probably don't cut it on stopping power.
3. If the wheel cylinders are bigger, you can replace the master cylinder and booster with 3/4 ton units.
4. You also need to check the hydraulic line sizes to make sure 1/2 ton and 3/4 ton are the same. That, I don't know.
5. You also need to replace the proportioninig valve. It will have different valving from the 1/2 ton unit. Note, it may also force you to change your hydraulic tube size.
Good luck, and don't drive behind me until you have it fixed.
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