Gentlemen,

2000 Expedition, EB, 5.4l, 4wd, auto

As mentioned in another thread, I recently blew a rear brake line - near the rear passenger side. I'm a bit surprised that at that time, the pedal went to the floor and I had zero brakes. Coasted to a stop.

Now, strangley, I did eventually find brakes. If I pushed the pedal to the floor, eeventually the fronts woudl still engage, although more weakly. But of course each time I did that I spewed fluid everywhere, so obviously this was only going to work until I emptied the master cyclinder.

But - I thought part of the functioning of the proportioning valve was to quickly lock-off a blown circuit due to low pressure so that the remaining circuit still functioned more or less. (Mind you, I can't even find the thing on my truck)

Now - I noticed that on the master cylinder there are two electrical connectors, one closer to the rear, one closer to the front. The front one is disconnected and taped off. Might have something to do with it?

In any case - am I misunderstanding what should have happened?

Thanks.

 

The function of the proportioning valve is to measure the pressure in both the front and rear brakes lines. If it finds a significant difference it will turn on the dash BRAKE light and possibly the ABS light as a warning of impending brake failure. (I'm not 100% positive on the ABS light.)

The PV in no way cuts off the flow of fluid even in the event of a brake line rupture.

 

The master cylinder has two chambers - you would only empty the chamber that supplies the rear brakes. The front brakes would still work (with the pedal burried in the carpet) but will have reduced braking effect without the rear brakes helping. So resist pumping the brakes and hold the pedal in the carpet.

The reason the brake pedal travel is excessive when you loose the rear brakes is because the two pistons (one for the front brakes and one for the rear brakes) within the master cylinder are normally separated by the brake fluid that controls the rear brakes. When all is working well and you step on the brakes, the piston at the rear of the master cylinder moves forward with the brake pedal and the forward piston is pushed forward by the brake fluid trapped between the two pistons. When you tramp on the brake pedal and the rear brake line lets go, as the rear piston moves forward, the brake fluid that usually is trapped between the two pistons goes out the rear leak. When the pedal is near the floor, the rear piston actually hits the front piston forcing it forward, and that allows the front brakes to engage.

I don't think there is one on these trucks (at least I have never seen one.) A porportioning valve (which is usually used as part of a "combination" valve) is more frequently used when you have front disk and rear drum brakes. The combo valve reduces the brake pressure to the rear drums, initiates the rear hydraulic pressure a split second before it allows pressure to the front brakes, and turns the brake light on if the front to rear brake pressure differential is out of spec. Since we have discs front and rear, they might just reduce the rear brake pressure by playing with the diameter of the primary & secondary pistons within the master cylinder, but I'm not sure about that.

One of those connectors is used to turn on the dash light if the brake fluid level is low. There is a magnetic switch within the master cylinder that monitors the brake fluid level in both the primary and secondary chambers.

The second connector picks up the brake fluid pressure, and IIRC, is used by the speed control as a backup to turn it off should the BPP switch fail.