No brake pedal pressure?

If you didn't catch, It, I fixed my previous reply. I'm working with someone else with a similar problem and I got confused. Sorry to say, it might happen again.

Correct. The pedal will move only as far as the lip seals pass the fluid return ports. At that point the fluid is trapped, and with nowhere to go, the pedal will be very high and solid. A trick now is to hold, but modulate your foot pressure on the pedal. By varying the pressure, a leaking seal may show up as the pedal slowly sinks to the floor. When you hold steady, high pressure, that can force the seals out to where they seal against the bore temporarily, thus hiding the fact they can leak.

If you do replace the master cylinder, don't worry about my comment about bending the lines up. I just do that so less fluid spills out. In fact, you can't do that with a lot of Fords because they have four lines instead of two. At most, you can bend two of them, but never all four. Bleeding is the same. Push the pedal down very slowly to prevent the air from going down with the fluid. Release the pedal quickly so the returning fluid washes the air into the reservoir. Even if you don't do that step, the air is not going to float down. It will work its way up into the reservoir in short order. That little air in the lines by the master cylinder won't even be enough to cause the pedal to be much lower than normal.

How do you know that? Are you going by feel or did you take the master cylinder off and measure the pushrod? What you're describing sounds like the pistons in the master are stuck partway down. That would really be weird, but if that's the case, it's due to that corrosion that builds up in there.

If you do have the master cylinder pulled forward off the booster, have a helper push the pedal, then watch the movement of the pushrod. See if there's a delay there. That pushrod that you can see is not actually part of the pushrod attached to the brake pedal. It's a two-piece affair with the rubber diaphragm in between. The two pushrod parts butt up to each other and form a mechanical connection when vacuum is lost due to a stalled engine. When it's working normally, pushing the brake pedal only opens a vent valve to let atmospheric pressure in on the backside of the diaphragm. That lets vacuum pull on the diaphragm which pulls the front pushrod out. If you don't see that front part move when the helper pushes the pedal, I would have to assume something came apart in the booster. That would also be a weird one.

Yes, they can be rebuilt, but not by you or me. That's a job for the people at the businesses that specialize in that.

If you look at the outer shell, you'll see a bunch of large tabs and slots that connect the front and rear together. You have to rotate one half counterclockwise a couple of inches to release the two halves. At that point you will go flying across the shop, (please don't ask how I know this), due to a very strong spring. You'll never get them back together. At that point the smoldering pile of ruins is only good for classroom discussion.

I'm sorry. I messed up again. That last reply was about boosters.

Master cylinders definitely are rebuildable, but doing so is not a good value. The kits cost almost as much as a professionally rebuilt unit with a warranty. The only time I would attempt this is when the kit is all that's available to get the customer back on the road.

Years ago, with cast iron master cylinders, it was standard practice to hone the cylinder to clean up the corrosion and gunk that formed in there. We can't do that anymore. Since the late '70s, master cylinders are made of aluminum to save weight. That corrodes very quickly. To prevent that, they get an anodized coating. That's an electroplating process. Honing can scratch that finish which will lead to rapid failure. The only thing you can do is to wash it with Brake Parts Cleaner and a rag. If that coating is already worn off, you'll see a lump of corrosion, like a soft rock, at the end of the bore. I don't know if they scrap those or if they replate them during the rebuild process.

This is a photo of the master cylinder for your truck. You can see the snap ring and the fully-released piston. I've also seen some with a wire ring holding the pistons in. Those require swear words to remove.

No, those lip seal are part of the piston assembly. If you see the piston is all the way back to the clip or ring, the seals are too. The only exception would be if the brke fluid is contaminated with any type of petroleum product. That makes the rubber lip seals swell and grow past the fluid return ports. The symptom would the opposite of what you have. By being blocked, fluid can't flow back up into the reservoir. That keeps the calipers lightly applied and generating heat. That heat migrates into the brake fluid causing it to expand. Since it can't expand into the reservoir, it applies the brake even harder, then more heat is developed. This keeps on until that brake is smoking and the vehicle is hard to move. The brake pedal will be very high and solid.

If the piston is fully returned, the next thing would be to pursue those two plugs in the ports to see if you have a high and firm pedal.

Push rods are not adjustable on most domestic models. They are on a lot of import models, but that is not a maintenance adjustment. It is set at the factory and is not meant to be changed.

It sounds like you're describing a mechanical problem inside the booster. I've never run into that on vacuum or hydroboost boosters. In fact, in over 40 years, I only replaced one vacuum booster and am aware of one other one that was replaced by someone else. I'm not aware of anyone having a problem with a hydroboost unit. Also, we don't rebuild them ourselves, so we don't teach that or even take them apart.

I'm going to get one of our other experts involved. He may have run into this before.