Brakes pedal traveling further than normal

Did you install a used or rebuilt master cylinder?

Forget the booster. That only makes the pedal easier to push. The place to start, assuming all the air is bled out, is to check the adjustment of the rear brakes. An inoperative self-adjuster will cause a brake pedal that gets progressively lower over time.

When half of the reservoir is out of brake fluid, there is a leak that must be diagnosed and corrected. Common places to look, especially if this occurred gradually, is seepage from the rear wheel cylinders. Also, check the steel line that runs to the back axle.

Did you bench-bleed it before it was installed?

You should not have to bleed at the wheels, but you might still have air in the master cylinder. Park on an incline with the front end lower, the stroke the brake pedal a few times.

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 car.

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 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.

It is good practice to never push the pedal over half way to the floor, although it will not 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 do not 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 does not show up until two or three days later.

When you are 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 will not be strong enough to pull the fluid past the lip seals.

I would try the bench-bleeding right on the truck, but instead of running a pair of hoses up into the reservoir, which you could do, I would just loosen the line nuts and watch for bubbles coming out.

The master cylinder does not care what pushes its pistons down for bench-bleeding. In this case, there is likely to be so much air in the lines that some of it will keep floating back up if you just gravity-bleed it. You may need to resort to pedal-bleeding with a helper. That will push the air out before it gets the chance to float back up. Do just one wheel at a time, and keep the other three bleeder screws closed. Be sure to close the bleeder screw you are working with before you holler to the helper to release the pedal. If you just hold your finger over the hole in the bleeder screw, air will sneak back in past the threads when the pedal is released.

Vacuum is pulling on the front side of the diaphragm. When you push the brake pedal, air is allowed in the back side so the vacuum can help pull the pedal. There is a filter screen to quiet that hissing, but if it continues, there could be a leak in the diaphragm. That is not common.

There are only two possibilities with that vacuum hose. It has a leak or it does not. If it has a leak, idle speed will be too high. If there is no leak, (including the booster's diaphragm), idle speed will be unaffected.

There are two other things to consider with a low brake pedal. The most common cause, other than air, is rear drum shoes that are not fully adjusted up. The next is the push rod coming out of the power booster. Chrysler did not have much trouble with that, but it does not hurt to check if it is adjustable with a threaded section on the tip. If there are some production tolerances in those parts, the push rod may be adjusted too short.

A less-likely problem to look at is to be sure the brake pedal is coming back all the way. If it does not, the lip seals in the master cylinder cannot take a new "bite" of brake fluid.

Be aware too that on some vehicles, what you think is a low brake pedal equates to the wheels are locked up and skidding on the road. This is especially common on GM vehicles. The brake pedal is fine, and the vehicle stops normally, but after that, it is real easy to keep pushing the pedal all the way to the floor. You have to drive those vehicles to know if the pedal travel is okay.

I do not know if these were used that far back but newer trucks and minivans use a height-sensing proportioning valve at the rear axle, for the rear brakes. It reduces brake fluid pressure to the rear when the vehicle is lightly-loaded, to reduce rear-wheel lock-up. If the vehicle is on a hoist, or supported with jack stands under the frame, the axle will hang down and mimic that light load. The proportioning valve may restrict fluid flow enough that the air in that line cannot be expelled.

A tricky one to find is a piston sticking in a caliper. The square-cut seal is supposed to stick to the piston, and bend slightly when the brakes are applied. When the brakes are released, that seal straightens out, and that retracts the piston a very slight amount so the brake does not drag. As the pads wear, the piston moves out further than the seal can bend, so it slides out through that seal. That is the self-adjusting feature of all disc brakes. If there is rust or dirt on the piston, or the chrome plating has lifted, the piston may fail to slide out and self-adjust. The seal will bend more than normal, then retract the piston too much. Next time the brake pedal is pressed, the piston has too far to travel, and the pedal goes too far to the floor.

The same thing can happen with loose front wheel bearings, but that will only cause a repeated low brake pedal after driving on the road. Road shock causes the rotor to wobble, and that pushes the piston back into the caliper too far. At the next pedal application, it goes too far again to push the piston back out. The clue to that is it only occurs after driving down the road, not when sitting in the shop.

To identify a sticking piston, use a flat-blade screwdriver to reach through the top of the caliper housing, and pry the piston back in. You should be able to do that rather easily with one hand. Next, watch that it slides back out smoothly when a helper works the brake pedal. This is how I always retract pistons when installing new pads. Some books show using a C-clamp to push the piston back in. That should never be necessary. A strong person can even run a piston in by hand. If the C-clamp is the only way to retract the piston, replace the caliper or rebuild it. We used to rebuild all calipers as part of a regular brake job, but today they do not cause much trouble. Many professionally-rebuilt calipers today are less expensive than the cost of replacing a piston with rust spots on it.