Brake pressure modulator valve

I suspect your van has rear-wheel-anti-lock brakes, (RWAL). That uses a valve assembly under your feet, on the left frame rail. That blocks and releases brake fluid to stop the rear wheels from locking up during hard braking. Failures of those are extremely rare, but if it did have internal leakage, it is already filled with brake fluid. It cannot cause the brake pedal to sink to the floor. There is no line or passage to allow fluid to flow back to the reservoir.

A low brake pedal is caused by air in the system, an external leak, or an internal leak inside the master cylinder. For this type of problem, a rear wheel cylinder is a good suspect for an intermittent leak, but those most commonly occur when the vehicle has been sitting for a long period, not so much while driving. Air in the system will cause a low pedal all the time, not once in a while like you described. That leaves internal leakage in the master cylinder, and an intermittently-slowly-sinking pedal is the most common symptom. The clue here is what is happening to the level in the reservoir? If it is dropping over time, look at the inner sidewalls of the two rear tires for signs of wetness from a leaking wheel cylinder. If the fluid level is not dropping, suspect the master cylinder.

You may also have a rear height-sensing proportioning valve with a linkage attached to the rear axle housing. Pickup trucks, minivans, and vans can have a wide range of loading, from empty to heavily-loaded. With a heavy load, you need more braking power to the rear wheels. Under light loading, this valve limits brake fluid flow to the rear wheels to prevent easy rear-wheel lockup. That valve is also filled with brake fluid, and unless it leaks, it cannot cause an intermittent low pedal.

If this is happening all the time, the red "brake" warning light on the dash should be on. By "internal leak" what I mean is the rubber lip seals in the master cylinder are ripped or worn, and when you push the brake pedal, the pistons don't push the brake fluid out to the wheels. Normally the pistons with their seals move about an eighth inch, just enough to pass a port that lets fluid run in from the reservoir, then the fluid ahead of those pistons is trapped. It has nowhere to go as you push the pedal further except out to the wheels. When those seals leak, it lets the fluid sneak back behind them instead of being pushed out. You moved the pistons, but not the brake fluid.

If you will allow me to get overly-technical for a moment, all outlet ports on master cylinders that feed rear drum brakes have a "residual check valve" in them where the steel line is bolted on. When you release the brake pedal, that valves stops the return of the brake fluid once it gets down to around ten pounds of pressure. To say that a better way, there is always ten pounds of fluid pressure in the lines going to the rear drum brakes. (This valve is never used for disc brakes because that would keep them applied). The return springs for the rear shoes can easily overcome that ten pounds of pressure, so the shoes still return completely.

There are two similar lip seals in every drum brake wheel cylinder, and there is a spring between them. The spring prevents gravity from causing either of those seals from falling over. The thinking is since the hydraulic system is sealed, when barometric pressure goes up considerably, in effect it could put the hydraulic system into a relative vacuum. By their very nature, lip seals block anything, (brake fluid, in this case), from going past them one way, but easily allow stuff to pass them the other way. With the system in a vacuum, air could sneak past the seals and get into the wheel cylinder.

Now, to be fair, I have never found it necessary to bleed a wheel cylinder as the only cause of a low brake pedal, and I have never replaced a master cylinder because of a defective residual check valve. This is part of the theory on system operation, but it leads to the point of value I'm building up to. Every spring when I bring my 1972 Challenger out of moth balls, the master cylinder's reservoir is empty and the car has no brakes. At first I found wetness on the inner sidewalls of both rear tires. That ten pounds of pressure had bled down, then the wheel cylinder's lip seals relaxed and allowed brake fluid to seep past them. Over winter that emptied the reservoir. It is the weight of the fluid in the reservoir that encourages the leaking, and once it is empty, there is little incentive for the rest of the fluid in the line to run out. That means there is air in just the first few inches of that steel line.

To solve this, I just refill the reservoir, slowly push the brake pedal, never more than half way to the floor, then let it release quickly. I repeat this about a dozen times, then go out for a drive. The more I use the brakes, the better the pedal gets, then it is fine for the rest of the summer.

By pushing the pedal slowly, new brake fluid is pushed down the line, and the air in it has time to float back up. When the pedal is released quickly, the fluid rushing back washes the air with it into the reservoir. This is the same trick I use to replace a master cylinder and never need to bleed at the wheels.

Any master cylinder over about a year old builds up crud and corrosion in the lower halves of the two bores where the pistons do not normally travel. When the pedal is pushed all the way to the floor, either due to improper bleeding procedures with a helper, or when surprised by a sudden leak, the pistons are run over that crud and that can tear the lip seals. That is your internal leak that allows the brake fluid to bypass the seal rather than being forced down to the wheels.

One piston pushes brake fluid to the front wheels, and the other one pushes fluid to the rear wheels. If you follow the two steel lines down about a foot, you'll see they go into a brass block on the frame rail. That is the combination valve. One of the three valves in it is the "pressure-differential valve". It just sits there with front fluid pressure on one side and rear fluid pressure on the other side. Both pressures are the same, so the valve does not move.

When one half of the system does not build as much pressure as the other half, that valve moves and turns on a switch that turns on the "brake" warning light. That is usually the same light that is used for the parking brake light, and, if you have a low fluid level switch on the master cylinder's reservoir, that is the third switch that can turn on that light. Pressure fails to build in half of the system when there is a leak in it, either an external leak that leaves a puddle on the ground, or an internal leak in the master cylinder, or when there is air in the system. Air will compress preventing pressure from building up.

Based on everything you have described so far, my vote is for the master cylinder having an internal leak. If I am right, you will not have needed to be adding brake fluid to the reservoir. If you do have to add fluid, look inside the rear tires and brake drums for signs of leaking brake fluid. By now the master cylinder is likely damaged from the repeated pedal going too far to the floor, but that may just be the final result after losing brake fluid through the wheel cylinders. If you see wetness, replace the wheel cylinders and the master cylinder. If you do not see wetness, replace the master cylinder, then if you lose brake fluid after the van sits for a long time, replace the wheel cylinders. The seeping brake fluid may be drying off as fast as it is leaking. You will not damage the new master cylinder, at least for the first year or so, by pushing the pedal over half way. Particles break off various parts, and all brake fluid loves to absorb moisture through the porous rubber hoses. That is what leads to the crud buildup, and that takes time to develop.

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Cheers, Ken