Brake pedal slowly sinks when at a stoplight

You already covered all the stuff I was going to discuss. It sounds like your new master cylinder is defective.

Your mechanic's comment about no special bleeding procedures required sounds correct as evidenced by your having a solid brake pedal for a week. Vans and trucks with rear-wheel ABS do not have any special procedures for bleeding. With four-wheel ABS, which is more common by 2004, some need a scanner to command the ABS computer to open two valves to allow two chambers in the hydraulic controller to be bled, but that has to be done with the two bleeder screws open at the wheels so the air can get out.

There is a simple trick when replacing a master cylinder that eliminates the need to bleed at the wheels, as long as no air has gotten into the lines. This is especially effective for saving time when the vehicle has a four-wheel ABS system that requires the scanner for bleeding. When calipers or wheel cylinders are replaced, it is standard practice to not allow the brake fluid reservoir to run empty when those parts are bled. That also insures no air can reach the ABS hydraulic controller, so again, no special bleeding procedures are required.

To address your slowly-sinking brake pedal, for it to do that, the brake fluid has to be going somewhere. Typically we think of a slow external leak, most commonly a leaking rear wheel cylinder. Calipers rarely leak. Rubber flex hoses normally burst, resulting in no brake pedal at all. Steel lines can rust through, but the slow leaks they cause usually turn into fast leaks pretty quickly. What all of these have in common is brake fluid is leaking externally and the level in the reservoir will go down. You have observed that is not what is happening. That only leaves internal leakage where the fluid is bypassing the seals in the master cylinder. When you push the brake pedal, after about the first inch of pedal travel, a port is blocked, then the fluid is trapped and has no choice but to be pushed down to the wheels. When one of the two seals leaks, that trapped fluid is able to sneak past it and go back into the reservoir. As it does, the brake pedal keeps going down slowly.

As a point of interest, this problem often occurs with older master cylinders right after the driver was surprised by a sudden leak, or by improper bleeding procedures. This does not apply to your van, and please do not blame your mechanic, but it is worth mentioning. After about a year, crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons do not normally travel. When you are surprised by a sudden leak, you end up pushing the brake pedal all the way to the floor, and that runs the two rubber lip seals over that crud and that can rip them. That results in the slowly-sinking brake pedal, like you have, and that often takes two or three days to show up. For that reason, when a shop gets a vehicle in for that leak, they will often include a rebuilt master cylinder in their repair estimate, rather than have to tell the owner later that more parts are needed. (We do not like having to do that).

This internal leakage is also often caused by do-it-yourselfers and inexperienced mechanics when they bleed the system with a helper pushing on the brake pedal. It is common for the helper to push the pedal all the way to the floor, and that is when the damage is caused. Professionals never allow the pedal to be pushed more than half way to the floor. In fact, most of us use gravity-bleeding. We do not use helpers or trust them with our jobs. Remember, this does not apply to your van because the master cylinder is too new. It still sounds like it is the same internal leakage that is occurring, but not because of anything the mechanic did wrong.

Here is an article that does a better job of explaining this problem:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

No particular brand. All are of good quality or the parts stores would stop carrying them. You can run into one where a new, inexperienced person put it together and made a mistake, but that is not very common.

If you are going to do this yourself, here is a trick I use on my own stuff. 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 van.

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 fifteen seconds to do that. You will 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 van by yourself, just keep the line nuts tight.

This wondrous trick might not work as well on Fords that have four lines at the master cylinder. What will happen is the air will bleed itself out to the reservoir if you give it enough time. Applying the brake pedal will not push air down the lines any further than you are going to be pushing brake fluid, so it will not get to the ABS hydraulic controller. All the air at the top of the lines will bleed into the reservoir in as little as a few pedal applications, or as long as a day or two. We cannot let a customer's car go that way, so we have to do this with the helper and opening the lines, but this has always worked for me when I am by myself.

Interesting video. We talk about this possibility all the time, but it is not that common to see.