No brake pedal pressure?

On most ABS systems, if air gets in the hydraulic controller, you need a scanner to get it out. There will be a routine under the "Brakes" menu for bleeding. It requires two people, one to press the brake pedal and one to open the bleeder screws. On most systems the scanner will tell you to open one front bleeder screw, then a rear one. The whole procedure takes only a few seconds once everything is connected.

When you do this job next time, place a stick between the front seat and the brake pedal to hold the pedal down about an inch. That will prevent brake fluid from running out of the master cylinder when a line is open. I have other tricks when you need them, including one that avoids the need to bleed at the wheels when replacing the master cylinder.

Let me know how far this gets you.

Nope. As soon as you run it in gear, the ABS Computer will see the wheels turning at different speeds. It will set diagnostic fault codes related to "wheel speed sensor performance" and turn the system off.

You have the right idea that the computer needs to activate the valves, but that requires putting the truck into a skid, and that requires running alongside the truck with a helper driving so you can open the bleeder screw. (I'm not recommending you try that).

The typical procedure is to bleed the brakes normally first, but that's just to run any air between the master cylinder and hydraulic controller down to that controller. That's where the fluid runs past the bottoms of two chambers while the air floats up to the tops of those two chambers and gets trapped. The scanner will typically tell you to open the right front bleeder screw while a helper holds the brake pedal down part way to the floor. (Even if you are able to, never run the pedal all the way to the floor unless the master cylinder is less than about a year old).

At this point the air in one chamber has been expelled and is in the line headed for that wheel. Close the bleeder screw, then the next step is to open the second bleeder screw, typically the right rear one. That clears the second chamber. Now all the air is past the hydraulic controller, and you finish up with normal bleeding at all four wheels. We've had a number of vehicles where no more air comes out after the scanner part of the procedure was done.

Everything comes in pairs. I've noticed this since the 1990s at the dealership. Today a second person asked if they could bleed the hydraulic controller with the car in the air. I won't see this same question for years now.

Let me know when you're done with this step and how easy or difficult is was.

Just went through this while helping a friend with a really miserable Ford Expedition engine. Ford left out two or three critical steps that turned three hours of frustration into six hours. You may have a similar situation where whoever wrote the instructions left out a step about opening a bleeder screw.

This isn't obvious at first, but it is once we think about it. The scanner talks to the ABS Computer and tells it to open two valves, usually one at a time, but once the air can be expelled, it needs a place to go. No brake fluid or the air in it can move until a bleeder screw is opened. There needs to be enough flow for the air to get past the two chambers and not get trapped in them again. That's just a couple of inches down the lines. Once the air gets into the lines leaving the hydraulic controller, it will flow to the wheels like when doing a normal bleeding process.

My friend and I each have a Snapon Solus Edge. Not better or worse; just more expensive than necessary. The screen tells us to open the right front bleeder screw, press the brake pedal, then press a button or screen prompt. There's buzzing and clicking that occurs too quickly to remember what's happening. Next, it tells us to close that bleeder screw, then open the right rear one and do the same procedure. It finishes up by telling us to continue normal bleeding at all four wheels, but by that time we already have a solid brake pedal.

The concern with pushing the brake pedal too far doesn't involve the ABS system. It pertains to all vehicles, but even a lot of experienced brake system specialists don't agree with this.

Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons don't normally travel. When the pedal is pushed to the floor, either during pedal-bleeding with a helper, or when you're surprised by a popped flexible hose, the rubber lip seals get run over that crud. That can rip them. That results in a slowly-sinking brake pedal, and that commonly takes two or three days to show up. To prevent that, never push the pedal over halfway to the floor unless the master cylinder is less than about a year old.

Even some service manuals tell you to push the pedal to the floor, but those instructions were written for a master cylinder that was just replaced. If something can't be handled by only pushing the pedal halfway, it won't be solved by pushing it further.

As for the pushrod in front of the power booster, those are rarely adjustable on domestic cars. They can be adjusted on a lot of imports, but that is not meant to be a service adjustment. It is set by the manufacturer of the booster then it's left alone after that. If someone does adjust it longer, you run the risk of it not allowing the pistons in the master cylinder to fully release. That would be the same as holding the brake pedal down an inch while driving. That runs the lips seals past the fluid return ports in the master cylinder, then that fluid is trapped and can't flow back up into the reservoir. Disc brakes will drag with the slightest brake fluid pressure on the calipers, and build heat very quickly. That heat migrates into the fluid causing it to expand. Since that fluid is trapped and can't release into the reservoir, it expands and applies the caliper even harder. That continues until that brake is smoking and the vehicle is hard to move.

Another way to look at this is if you were able to get a higher brake pedal by adjusting the pushrod longer, that is only overcoming or hiding the true cause of the low pedal. I was recently in a conversation with someone who insisted the pushrod was too short. My first thought was he was handed a new master cylinder for the wrong application. That's when he added the port sizes were different and he had to buy a pair of brass pipe adapters. That was the clue as that should never be necessary.

For my last comment of value, (for now), look at whether your master cylinder sits parallel to the ground or not. A lot of master cylinders on GM vehicles sit really high in front and air can get trapped there. That air comes out when bench-bleeding it in a vise on the workbench, but it may be more difficult to get out once it's on the vehicle. You'll see the port is cast into the body above the center of the bore, and it's way up in front for the front line. I have a trick that lets me bleed after replacing a master cylinder without having to bleed at the wheels, and that works here too. Loosen that soft metal line nut, then have a helper push the brake pedal partway down very slowly. It should take a good 15 seconds to get halfway to the floor. You'll see air bubbles spurting out around the threads. Tighten that nut, THEN holler to your helper to release the pedal quickly.

If the pedal is pushed down too fast, air can follow the brake fluid down the steel line. By pushing slowly, those air bubbles keep floating back up as the fluid goes down. By releasing the pedal quickly, the fluid rushing back into the reservoir will wash any air bubbles up there with it.

What I'm suggesting is your scanner is missing the written step telling you to open a bleeder screw. With every one I've done, the engine doesn't need to be running. Doing so just gets the power booster involved making it easier to push the pedal.

Also, with our scanners, it always has us do this in two stages, first with just a front bleeder open, then with just the rear one open. If your scanner only tells you to do this once, I would have to assume they intend for you to bleed with two bleeders open at once, or they want you to back out of the test, then go in and do it a second time for the rear bleeder screw.

I've only used two scanners for brake bleeding. Most were done with a Snapon Solus Edge, but for all of my older vehicles, I have a Chrysler DRB3. I bled the rear brakes once on a '94 Grand Voyager after replacing a rusted steel line, but that was so long ago, I don't remember what the steps of the procedure looked like.

Don't panic if it takes a little time for me to reply. I'm here once a day for a few hours. I'm pretty sure we're going to get this solved.

If no air made it down to the hydraulic controller, you can get the air out of the lines leaving the master cylinder by working the brake pedal multiple times. This works even better during a drive cycle. You have engine and road vibration to help bounce the air bubbles back up.

I have to do this every spring with my '72 Dodge Challenger. All drum brake systems have a "residual check valve" in the master cylinder's port for the drum brake wheel cylinders. That valve holds ten pounds of pressure on the brake fluid to prevent air from sneaking past the rubber lip seals when barometric pressure goes up overnight. Over the winter, that ten pounds bleeds off, then, with no pressure holding those seals tight to the cylinder, brake fluid can seep out and make a puddle on the floor. I'll find the reservoir empty and some air in the lines going down to the combination valve on the frame rail.

All I do is refill the reservoir, stroke the brake pedal a few times, then go out and drive it. Each time I press the brake pedal, fluid runs down the lines. When sitting at a stop light, the air gradually floats back up. When the brakes are released, the fluid rushing back washes the air with it up into the reservoir. After that, the brakes are fine for the rest of the season.

Another way you can do this with a helper is to loosen one of the soft metal line nuts at the master cylinder, then have your helper slowly push the brake pedal about halfway to the floor. It should take about 15 seconds to do so. If there's any air in that line, it will be pooled by that connection. You'll see air bubbles spurt out, then brake fluid. Tighten that nut, THEN holler to the helper to quickly release the pedal. Do that a second time, and even a third time as long as you see air coming out. The few bubbles that might remain will work their way out during driving. Do that procedure for the second steel line. This is especially useful for GM vehicles where the master cylinder sits at a wild angle, higher in front. There's no way air is going to go down to be drawn into the reservoir. Loosening the lines is almost a requirement to get that trapped air out.

If air made it down to the ABS hydraulic controller, you'll need to use the scanner again to free it from the two chambers. While I don't recall ever seeing this instruction, you may need to do the scanner procedure by opening bleeder screws on the left side. It shouldn't matter, but there's a lot of different hydraulic controller designs, and they might have different bleeding requirements.

I have three comments of value today. First, do not use any grease anywhere close to brake fluid. That includes penetrating oil to loosen rusty fittings and connections. If any petroleum product gets in the brake fluid, it will contaminate the entire system. The only proper repair for that is to replace every part that contains rubber parts that contact the fluid, flush and dry the steel lines, then install all new parts and fluid. If any part with rubber is not replaced, the contamination will leach out of it and recontaminate the new fluid and parts. This includes the rubber flex hoses, calipers, wheel cylinders, ABS controller, master cylinder again, and the bladder seal under its cap. A lot of trucks and minivans also use a rear height-sensing proportioning valve. Those have rubber o-rings so it must also be replaced. For all vehicles without anti-lock brakes, there's also a combination valve under or near the master cylinder, usually mounted to the frame rail. It too has rubber o-rings.

Every year I did a demonstration where I put a wheel cylinder lip seal in each of two beakers filled with fresh, clean brake fluid. I added one drop of power steering fluid to one of them, then let them sit for a week. By the end of that week the contaminated seal had grown by about 20 percent and was soft and mushy. Felt real slimy too.

Air will not enter the system through the threads by the bleeder screws. The only way that could happen is if you're pedal-bleeding with a helper, and instead of closing the bleeder just before the pedal is released, you just hold your finger over the hole in the screw. While it's possible a little air could try to sneak in, it's just as easy for more brake fluid to run into the master cylinder from the reservoir as the pistons move back. What little air might get in would never be noticed in the pedal.

What's much more common is to see a steady stream of tiny air bubbles when using a vacuum bleeder. They usually have clear hoses so you can see what's coming out. Air in the system will come out as long solid bubbles. Air getting drawn in past the threads will be those really small bubbles that never stop or slow down. Those never make it into the caliper or wheel cylinder.

Second, if we assume there's no air in the system, that leaves two things we haven't considered yet. This doesn't apply to your truck, but for the benefit of others researching this topic, if the rear uses drum brakes, shoes that are out of adjustment will require excessive brake pedal travel. That's needed to run the shoes out until they contact the drums, then pedal pressure will start to be developed. The huge clue to this is the pedal will become higher and firm when it is pumped rapidly a couple of times. Each time the pedal is pressed, the shoes move out toward the drum, but when the pedal is released, it takes a considerable time for them to retract under spring tension. They'll still be extended partway out when the pedal is pressed again, so each time the pedal is pumped, less travel is needed. As soon as the pedal is released for a good five seconds or more, the shoes will have time to fully retract, then at the next pedal application, it's back to needing too much travel again.

Older GM and Ford vehicles with rear disc brakes used calipers with the parking brake built into the pistons in those calipers. Those came with all kinds of special problems. One was the need to fill the pistons with brake fluid before installing them. Once installed, there is no way to get fluid into them and get the air out. The next problem is unlike with front calipers and their ease of self adjusting by pumping the brake pedal, rear calipers are adjusted by multiple applications of the parking brake. If that isn't done, the pistons will never run out like they do in front calipers. They'll extend when the pedal is pressed, but then they'll retract again when the pedal is released. A common clinker is the parking brake cables are rusted tight. When they can't be used, the calipers can be adjusted by crawling underneath and working the levers on the calipers with a large pair of pliers. That can be done with the wheels on the vehicle or with them off when the calipers are being installed.

Related to all designs of calipers, check for binding mounting hardware or slides. As the pistons extend, they pull the caliper housing inward to pull the outer pad to the disc. If the mounting slides are rusty, or mounting pins have areas where the chrome plating has lifted, those can cause the caliper to not slide freely to adjust. The piston will move excessively to apply the brake. That translates into further travel of the brake pedal.

Finally, the one thing that's left is what I suspect has happened. There's at least four different master cylinders listed for your truck. They have different cylinder diameters for different applications. It's customary to incorporate some significant difference so the wrong one can't be installed. The most common is the port size. In this case they're all the same, so it appears any size master cylinder can be installed. The cylinder diameters listed are 1.25", 1.338", 1.438", and 1.456". I'm betting you have a 1.25" diameter bore when the application calls for a larger one. With the smaller bore, you'll need to push the brake pedal further to get the required volume of brake fluid.

There's almost 1/4" difference in bore sizes. That is huge when designing a system that's balanced front-to-rear, and with a good compromise between pedal travel and pedal effort.

GM lists different part numbers but I can't find any numbers on the castings to compare. That leaves physically measuring the diameter of the bore. You might be able to do this with a tape measure. If that doesn't provide enough accuracy, you'll have to pull the piston assembly out, then measure the inside diameter of the bore.

Let me know what you find.

My fault. I confused your truck with another one with a similar problem. That one has a new master cylinder.

This IS the time to consider the master cylinder has an internal leak. Given the age and the amount of bleeding that has been done. There's a real likely chance the seals have been ripped from the crud that builds up in the lower halves of the bores. One time-consuming way to verify this is to make plugs that can be screwed into the ports in place of the steel lines. The brake pedal will still move quite a bit if the seals are leaking.

Another clue is when you open a bleeder screw and have a helper press the brake pedal, very little fluid will come out. It can be hard to know how much fluid to expect, so...

Another trick is to use that stick to hold the brake pedal down about an inch to block the fluid return ports, then use a large flat-blade screwdriver to pry a caliper piston back into the housing. Normally you can do that with one hand, but if you can do it with the brake pedal partway down, fluid has to be leaking past the rubber lip seals in the master cylinder.

Throughout this ordeal, we haven't really talked much about those calipers. Some designs use a pair of long, chrome plated pins that slide through rubber tubes. If the chrome has lifted on those pins, rust forms underneath. That can allow a caliper to move due to flexing of the rubber tube rather than sliding on the pins to self adjust. The flexing tubes can pull the caliper back away from the rotor, thereby preventing it from remaining in that adjusted position. It will take an excessive volume of brake fluid to pull the caliper back to the rotor before pedal pressure can build up. To identify that, watch each caliper as a helper works the brake pedal. You should see very little movement from the caliper. Once the brake pedal is halfway to the floor, you should not be able to wobble the rotor by hand. If you can, suspect the master cylinder. Even with air in the lines, the caliper should clamp onto the rotor with enough force that can't be overcome by hand pressure on the rotor.

If you do replace the master cylinder, you don't have to bleed at the wheels. Here's a trick I've used since the 1980s:

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 vehicle.

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 15 seconds to do that. You'll 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.

It's good practice to never push the pedal over half way to the floor, although it won't 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 don't normally travel. Pushing the pedal to the floor, like most competent 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 doesn't show up until two or three days later.