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I've had that problem for ages. You have ABS? I'm pretty sure I determined that the ABS system has no proportioning valve external to the ABS modulator. Very simple system (forgive me if I don't nail it, I haven't fooled with the brakes in a while): Brake fluid reservoir, through master cylinder, into ABS modulator, out to each of the brake circuits (two for front, one for rear).
Assuming you have ABS, you have a two circuit master cylinder. One can be leaking (so I'm told) while the other works just fine. That is, the seals in the front master cylinder channel can be passing fluid internally, while the rear is fine. I replaced my master cylinder in an attempt to fix the problem; I have noticed no difference.
Always be sure your calipers and hoses are in good order before checking for more serious (costly) problems. I replaced mine - no help.
In an attempt to have firm brakes which apply evenly, I have replaced all parts of my brake system but one: the ABS modulator. One day I will get a spare and swap it in to see if that helps, then hack open the original and see what's wrong. Until then, my brakes suck and I drive very carefully.
P.S. I solved part of the problem (uneven braking, front-to-rear) by replacing rear brake pads. I had bought ceramic pads for the rear but continued to use semi-metallic in the front. Rears gripped harder. But I still suspect my brakes do not apply as hard in the front as they should.
Good luck.
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