My opinion, I have never like Firestone. I worked dor them for two weeks and said F this huge corporate policy BS.
Anyways.
They have attempted to fix the problem. Many times, to little or no success. Go back to the shop and rasie hell. File a report with the BBB.
What they are doing is the exact same thing that everyone says stealerships do. Its not right (I know, the world is not about what is fair and right), but there are some steps you can do to potentially get money back or the repairs for free.
Do not worry about making a scene, your not going to do buisness with them after this it sounds like.
I have never seen a faulty break switch cause jerking. The brake switch is just a switch for the brake lights and an input to the ecu to cancel cruise.
Brake master cylinder, possibly could cause hard or soft brake pedal. I guess it could cause jerking if the entenal bore was jacked up and jerked while under braking (which is not what you said is happening).
600 psi on what? What did they replace, the ABS pump?
This could cause jerking if a sensor was detecting slipage, and the ABS was activating. But you said there was no noise when the truck would jerk.
I wonder if you have a bad ABS encoder ring? This could cause an issue, similar to random activation of the ABS. Maybe?
One thing that keeps poping into my head. Was your brake master cylinder leaking by chance? It is common for them to leak past the rear seal, and leak into the brake booster. This can cause a buildup of pressure which can cause the brakes to be under constant pressure.
I had this happen on my 00 Outback. I replaced my brake pads, and they wre eaten up in 5k miles. Spun all wheels, which were tough to spin. I replaced all calipers, rotors, and pads. Ate those up in 1k miles. Thinking I got bad calipers from Napa, I replaced them all again. Ate those up in 1k miles. My friend and I adjusted the brake pedal, no change. My friend had the idea, lets put some spacer on the brake master. Bam! No more jerking, no more eating brakes, and the pedal felt better too. So I replaced the brake booster. Never had a problem again.
Then I sold the Outback for the X. Saw it had a leaking brake master, and I replaced it and the booster at the same time.
I have replaced boosters before on Hondas and Toyotas that were acting similar to my problem.
Maybe your booster is faulty. The way I know to test it,
Engine off, press the brake pedal several times till it gets hard.
Whill pressing, and it is not going down, start the engine. The pedal should decend to the floor smooth and at a constant rate. Maybe half a sec after the engine starts.
If it decends, before you start the engine, you have a brake fluid leak or air in the system.
If it does not decend after you start the engine or it takes a long time, you have a vacuum leak some where.
After the engine is started, let the vacuum build for a sec or two. Press the pedal several times. You may hear a whoosh sound every time you press the pedal. If you press and hold the noise should stop, if you still hear it, the booster rear diaphragm is most likely blown. It is common to hear some air whoosh while pressing the brake pedal, but if it is louder than normal, this may bed the diaphragm.
Sorry, this was a long one.
Hope you get your X fixed soon.