That's the classic symptom of a failing master cylinder. You're still holding pressure on it, but you're modulating that pressure.
The clue to this "internal leakage" is you're not losing brake fluid. This article explains it better:
Here's a trick I've used since the '80s to avoid having to bleed at the wheels.
When you replace the master cylinder with two steel lines, loosen the line nuts a little, remove the mounting bolts to the power booster, pull the master cylinder forward, then use it as a handle to bend the steel lines up a little. That will keep the fluid from running out of the lines.
Remove the two lines all the way, then remove the master cylinder. Brake fluid eats paint, so be careful to not allow any to drip onto the truck.
Screw the two lines into the new master cylinder that has been bench-bled, then use it to bend those lines back down to their normal shape. Bolt it to the booster, then snug one of the line nuts. Have a helper slowly push the brake pedal half way to the floor. It should take about 15 seconds to do that. You'll see bubbles coming out by that nut. Snug the nut, then holler to the helper to quickly release the pedal.
Do that a second time, and perhaps a third time, until you see only clear fluid with no bubbles coming out, then do that for the other line. By pushing slowly, fluid will get pushed down the lines, and air will float back up. By releasing the pedal quickly, the fluid rushing back will wash the air back up into the reservoir with it. This can even work when working on the car by yourself, just keep the line nuts tight.
This wondrous trick might not work on Fords that have four lines at the master cylinder. You still don't have to bleed at the wheels, but it can take a little longer for all the air bubbles to be expelled into the reservoir.
It's good practice to never push the pedal over half way to the floor, although it won't matter with a rebuilt master cylinder. Once they get to be about a year old, crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal to the floor, like most do-it-yourselfers do, runs the rubber lips seals over that crud and can rip them. That results in a slowly-sinking brake pedal, and that often doesn't show up until two or three days later.
When you're replacing calipers or wheel cylinders, a lot of people make misery for themselves by allowing the master cylinder to run empty, then they have to bleed the entire circuit. Often a scanner is needed on anti-lock brake-equipped vehicles to open some of the valves so the chambers can have the air expelled. To avoid that, use a stick between the seat and brake pedal to hold the pedal down an inch or two. Gravity won't be strong enough to pull the fluid past the lip seals.
Also, when you do bleed at the wheels after replacing the master cylinder, any air in the lines gets pushed down to the wheels, and if the vehicle has anti-lock brakes, it's common on most designs for that air to become trapped in the hydraulic controller. That's where you need the scanner to do the bleeding procedure. I find it much easier and faster to just avoid pushing that air down there.
Here's another article you find useful:
Wednesday, March 10th, 2021 AT 11:29 AM