2003 F250 7.3 4x4 Auto. I've been trying to fix a weird braking issue for a while including taking it to a mechanic who threw $1400 of parts at it to no avail.

The Problem:
-The pedal feels normal under light to medium braking, but under hard braking it gets hard and feels like i'm being pushed by a 7k pound trailer
-With the truck turned off, the pedal is hard and tight like it should feel
-The brakes are getting barely enough power that only the rear brakes are barely trying to lock up and activate abs
-I believe everything started when my OEM steering pump went out but it wasn't immediately noticeable because the braking felt normal under light to moderate braking, if compared to my dads 2000 7.3.
-First time i noticed it was when towing a trailer a few months after the pump replacement, went to stop and it pushed through them easily.

Fixes Tried By Me:
-The OEM pump was replaced by an oreilly pump, but didn't notice much difference at the time
-Maybe 4-6 months later my front left brake caliper was dragging, but turns out the wheel bearing went out and i think it was activating the abs, making me think it was the caliper so I replaced both bearings and both calipers anyway
-I started testing brakes regularly after that and started trying to fix the issue, started with a new master cylinder with no change
-After that i put in another new pump, Lares from Rockauto this time with no change
-Then i thought the only thing left was the hydroboost, bought a new Motorcraft unit and that made no change either
-Did a ton of bleeding including the forscan ABS bleed multiple times following fords bleed procedure

After that i was done mindlessly throwing parts at it, so I took it to the highest rated diesel mechanic in my area hoping they'd have more equipment to test and figure out the real issue.

Fixes the mechanic tried:
-They did an ABS bleed and called me and told me it was fixed, so made the drive and there was zero change to what i originally brought it in for.
-After that, they replaced the calipers, then once again called and told me it was fixed, well that did absolutely nothing as well
-The third trip, they replaced the ABS HCU and said the new hydroboost was leaking so they put a new one and I returned the one i bought. They called, said him, a tech and the owner all drove it, said its definitely fixed, so i showed up again and there was zero change to my original problem
-The final trip, i asked if they had a flow/pressure tester for the pump to get actual numbers and they didn't have one, said he'd put a ford pump on it at no cost to see if it fixed it, it made zero difference so i brought the truck back home after $1400 and 2 months.

I'm at a loss now. I don't want to throw any more parts at it without real diagnosis. I think they all thought it was "fixed" because it felt fine at light-moderate braking but they never went full pedal force braking where it was weakest, but it shouldn't be my responsibility to tell them how to fix a truck.

Has anybody fought a similar issue that could give me a next direction?

 

If you want to diagnose it will take some equipment. Cheapest and easiest would be a brake pressure gauge, they screw into the bleeder port and you can measure the actual pressure at each wheel. Then you can measure the ABS output and then the MC output. You can make all of the pressure you want but if it's not getting to the wheel it's not doing any good. The next would be to measure the actual pressure of the PS system which is a little more complicated to tie into the PS. Of course with all of the different hands working on it I'd want to check for any human error like crossed lines or kinked lines.
Simple test is truck off pump the brakes a few times discharging the accumulator. Then start the truck with your foot on the brake pedal, when it starts you should feel the pedal raise when the hydroboost gets pressure.

 

Have you replaced the soft lines in the braking system?

 

The rubber hoses do break down after so many years and the rubber will 'flap' inside the hose and slow the flow of brake fluid in one direction or the other. Have you changed the rubber hoses out? This will also cause the calipers (if hose flaps wrong) to 'hold' the brake pads to the rotor and cause wear and sticking when you have released the brake pedal.

 

Quick test: Pull the ABS fuse and see if braking power returns.

 

Thank you everyone for the suggestions.

Both front caliper soft lines have been replaced as well as the rear caliper soft line. The axle soft line hasn’t.

I did the brake pedal test first, pumped it a few times and the pedal stayed firm. I cranked the truck, it fell slightly during cranking and then got a bit softer when started.

After that I did the abs fuse test. I unplugged the abs fuse and the rear calipers were locking up a ton, but the pedal wasn’t hard anymore. I couldn’t do that test more than a couple times because it was locking the rear up so bad that I was getting axle wrap and shaking.

Correct me if I’m wrong, but my thinking is that there’s some sort of blockage or pressure differential in the front calipers that the abs is fighting so hard to keep the rear brakes from locking up, it’s giving me the hard pedal and weak overall braking. I ordered a brake pressure test kit and that should be in soon.

 

I know you said you replaced the master cylinder but have you checked fluid pressure at the output port of the master cylinder for the front brakes?

Perhaps there is a short in the front ABS sensor lines. Monitor ABS PIDs with FORScan and see of they are reading the right values.