Does the TC unlock when you hit the brakes? And will it re-lock when you take your foot off the brakes? With OD disengaged of course.
YES. But it depends on the PCM calibration. Some calibrations commanded the TC to unlock every time the service brake pedal is touched, regardless of OD engagement or disengagement, and irrespective of speed.
I have one of those calibrations. And I have a TC lock/unlock indicator LED that provides visual confirmation when the TC is locked, unlocked, or in the process of locking (the pulse width modulation command circuit flutters the LED indicator light when engagement is in process). These calibrations are typically earlier model years, model year 2000 and below.
On the other hand, some PCM calibrations factor in vehicle speed before unlocking the TC when the service brake is touched, and do not unlock right away if the vehicle speed is above approximately 40 mph, even if the brake pedal is touched. On these calibrations, if the vehicle speed is below X (where X is around 35 mph) and the brake is touched, then the TC will unlock. These calibrations are typically found in later model years, 2001 and above.
Nowadays, one cannot go by earlier or later model year to determine calibration. Too many people run chips and/or tunes where the calibration is modified. Even where folks report that they are running on their so called "stock" tune setting in their Hydra or flip chip or programmer, that "stock" tune is often a simulation of a stock tune, not the actual stock tune of the base pcm. And the simulated stock tune can be based on a more popular stock PCM calibration that has been found over the years to be preferable.
And one of those calibration preferences is preventing the TC from unlocking any time the service brake pedal is touched. The way Ford transmission engineers initially programmed TC unlocking on brake application has frustrated many Super Duty owners who, while descending a grade, just want to tap the brake briefly for a touch, say when someone in front of them turns on their turn signal to switch into their lane. They don't want to suddenly gain MORE speed as their transmission free wheels into a fluid coupling while they are barreling down hill with a trailer at 55 mph.
Tuners know this, and have either tuned that annoyance out of their simulated stock tuning, or have selected a later Ford calibration upon which to base their entire tuning platform. And this is what makes BigB's question hard to answer based on other user reports with their model year. Because unless the person reporting knows the base calibration of their tune (even in the stock setting), or knows whether or not their PCM has ever been changed or flashed (which might rely on continuous ownership of their truck since brand new), it isn't possible at this point entering into the 20th year these trucks have been in service to know.
But 19 years ago, we DID know, and we did comment, on this very forum, how much it sucked having the TC unlock immediately on service brake application. Aftermarket products (ie, Banks Smart Lock), home spun solutions (ie TC lock/unlock switches), and numerous tunes were created to surmount this problem until Ford finally fixed it.