Brakes go to floor

If you have a vacuum-operated power booster, it had better have air in it. You do not expect it have brake fluid. Power boosters, whether run by vacuum, power steering fluid, or brake fluid, only make the brake pedal easier to push. They have nothing to do with how far the pedal moves.

If the brake pedal goes too far to the floor, the first suspect is air in the hydraulic system, but that assumes you did something that allowed that air to get in there. Typically that means calipers or wheel cylinders were replaced, and the master cylinder was allowed to run empty. There is an easy way to prevent that.

If the vehicle has anti-lock brakes, often it is necessary to use a scanner to command the computer to open some of the valves so the air can be expelled.

The second common suspect is someone resorted to pedal-bleeding with a helper, and the master cylinder was damaged. This is a mainly a problem with do-it-yourselfers. Crud and corrosion build up in the lower halves of the two bores where the pistons don't normally travel. During pedal-bleeding, inexperienced people ram the brake pedal all the way to the floor in some misguided thinking that it is necessary to get the air out of the system. Doing that runs the rubber lip seals over that crud and rips them. That causes internal leakage, and that may not show up for two or three days. Internal leakage causes the brake pedal to sink slowly to the floor at first, when steady pressure is held on the pedal, but eventually it causes a failure to build any pressure when the pedal is pushed. Experienced mechanics avoid this by gravity-bleeding with no helper. If you do resort to using a helper, it is important to never push the pedal over halfway to the floor. This is even more important on GM front-wheel-drive cars. Pushing the pedal over half way will trip a valve in the master cylinder, then you will not get any fluid flow to one front wheel and the opposite rear wheel.

Very informative. Nice work CARADIODOC!

Some people, and even in some service manuals, they will tell you tripping that valve can be avoided by bleeding in a specific sequence. Some mechanics do the right rear wheel first because it has the longest line and clearing it of air removes the largest percentage of that air with the first wheel. This is supposed to make bleeding the other wheels easier or faster, ... I guess.

Some people bleed the left front wheel first because it is the shortest, and that is the fastest way to make one wheel solid. That removes all the "compressability" from one quarter of the system.

Even the service manuals often tell you to bleed a certain wheel first to avoid tripping that valve, but if you think about this logically, the apparent reason for having that valve is to block fluid flow to the half of the system that develops a leak. Who knows which wheel that will be? If the valve would not trip if you bleed the right rear first, for example, it also would not trip if the right rear line sprung a leak, so what would be the point of having it?

For a related comment of value, crud and corrosion build up in the lower halves of the two bores in the master cylinder where the pistons don't normally travel. Pushing the brake pedal over half way to the floor runs the rubber lip seals over that crud and can rip them. That results in a slowly-sinking brake pedal, and that may not show up for two or three days. Where many do-it-yourselfers get into trouble is they ram the brake pedal all the way to the floor when bleeding with a helper, or they push the pedal over half way to run the pistons out of the calipers after they were pushed in to make room for new brake pads. Either procedure can cause one front caliper to self-adjust first and start to build fluid pressure as the pedal is pushed further. The other front caliper hasn't gotten there yet, so no pressure builds up in that circuit. To the valve in the master cylinder, a failure to build pressure looks like there's a leak, and that results in that valve tripping to block the port.

Most front-wheel-drive cars use a "split-diagonal" brake hydraulic system. That means the left front and right rear are on one circuit, and the other two wheels are on the other circuit. That insures that with a failure in one system, you will always have one working front brake. That is important because about 80 percent of the car's weight is on the front. If the car had the older-style front / rear hydraulic system, and the front system failed, they'd find you in the next county with skidding rear tires before you would be able to stop!

If you ever drove an older car with one disabled front brake, which I've known youngsters with no money for repairs to do, you would see that applying only one front brake would tear the steering wheel out of your hand. Cars were almost impossible to drive that way. You need the opposite tugs from both wheels, and the steering linkage to connect them together, to offset each other. With today's front-wheel-drive cars, the engineers have modified some of the non-adjustable suspension geometry to overcome that brake pull. On Chrysler products, that has been so well perfected that the only way to know there is a failure in half of the system is the red warning light turns on. On most other brands, the only clue you might observe is a tiny wiggle in the steering wheel when the brakes are applied. There is no hard pull to either side.

Okay; enough for today's story hour. You asked how to reset that valve. To my knowledge, GM is the only manufacturer that uses it. I already described what trips the valve and what some people think it takes to avoid tripping it. Some people also think it can be reset by bleeding the other circuit. It can not. Some people replace the master cylinder, then they trip that valve in the new one by pedal-bleeding with a helper. (Bleeding at the wheels is not necessary when replacing a master cylinder). Regardless of why it tripped, the only way I have ever found that will always reset the valve is to loosen the cover on the brake fluid reservoir, go to one of the wheels that is not flowing any fluid, open that bleeder screw, then give it a quick, short burst of compressed air. The goal is not to send air all the way up to the master cylinder. It is only to move the brake fluid a few inches up the line to push the valve open. To avoid getting too much air in the system, I use a rubber-tipped air nozzle, then I give it a quick slap with the side of my hand. Let it gravity-bleed after that. Fluid will be running out within ten to 20 seconds. Close the bleeder screw, and when you're done bleeding at all the wheels that needed it, "irritate" the brake pedal by hand a little. That will push any remaining stuck air bubbles into the calipers or wheel cylinders. Open each bleeder once more for a couple of seconds to expel those bubbles.

If you still need to run the pistons out of the caliper housings after installing new pads or rotors, be careful to not push the brake pedal over half way to the floor. This applies to all master cylinders for that corrosion I mentioned, unless they're less than about a year old, and it applies to GM vehicles to prevent tripping the valve again.