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There is a proportioning valve that is supposed to limit the amount of fluid to the front or rear brakes since they require different amounts of pressure to operate. If the rear brakes are locking up and the fronts are hardly working, the proportioning valve is probably stuck one way or the other.
Do you have a brake light on? It usually triggers the brake light when this is acting up.
Also is your truck equipped with ABS?
Is your truck 4wd or 2wd?
Does it sound like the condition in this TSB is happening?
Does it look like the front brakes are applying when the vehicle is not moving? In other words does it seem like the front brakes don't work only when moving?
Have you bled the hydraulic control module with a compatible scan tool? The only way to get it to bleed is with a scan tool. You could try to drive it on a dirt road and do a couple of skid stops to try to get it to self bleed, but it's not the best way to do it.
If you have, I would probably replace the proportioning valve. The problem with that is that they are really expensive and then you will still have to bleed the system with a scan tool. Here is the part number from Ford: 1L5Z2C219EA
The proportioning valve is a combination valve that is just below the brake master cylinder.
It looks like on the 2002 6 cylinder with 4 wheel abs the proportioning valve and the hydraulic control unit, the picture you sent, are one unit. On the 4 cylinder they have an option of 2 wheel anti lock which has a separate proportioning valve that runs off of the bottom of the master cylinder.
I assume from your picture that you have the 6 cylinder. You will likely need to replace the hydraulic control module, but keep in mind it will need to be bled with a capable scan tool.
I think it probably just needs a bleed with a scan tool.
Let me know if you need any more help.
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Let me know if you need additional information. I would love to help if you still need assistance.
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