Bought used Master cylinder part and pedal is MORE spongy, why?

A brake master cylinder is one of the few things you never buy used. Most salvage yards won't even sell them. After they get to be a few years old, crud and corrosion build up in the lower halves of the bores where the seals don't normally travel. When anything happens, such as a leak, that lets you push the pedal more than half way down, those seals can get ripped on that crud, then you have junk. There's no point in buying a used master cylinder that you know is going to suffer the same fate. Even if it wasn't bad before, by bench-bleeding it before installing it, you run those seals over the crud and cause the damage. Many smart mechanics even warn customers they will likely need a rebuilt master cylinder when their car has a leak from a flex hose or rusted steel line.

The place to start is by getting a rebuilt master cylinder, bench-bleeding it, and if you are careful, you don't even have to bleed at the wheels.

First, loosen the steel lines at the master cylinder just enough that you can turn them later without having to tug really hard. Next, unbolt the master cylinder from the booster. (The booster won't cause the problem you're having). Use the master cylinder as a handle to bend the steel lines upward just a little. That will prevent brake fluid from running out while those lines are disconnected. Remove the lines, then thread them into the new master cylinder. (Don't remove the plastic hoses that come with the master cylinder until after it is installed on the car). Finger tight is sufficient for now. Push it down to bend the lines back to where they were, then bolt the master cylinder to the booster. Have a helper push the brake pedal down slowly. It should take him about 20 seconds to go half way. As he does that, you'll see air bubbles coming out at the steel lines. When he hollers that he's half way down he must hold the pedal there while you tighten the steel line nuts, THEN he can let the pedal spring back quickly. When he does, any air bubbles stuck in the line will wash up into the reservoir along with the fluid. You'll want to do that two or three times, loosen the lines, watch for bubbles, tighten the lines, release the pedal. It may work better if you do one line at a time. Keep the second one tight until it's time to do that one.

By using that method there is no reason for air to go down to the wheels. It always floats back up to the high point which is the master cylinder. The exception would be if there's air in the lines now from your previous work.

The booster doesn't even do anything when there's no vacuum from the engine not running. All of them have a two-piece mechanical link that never operates unless there's a loss of vacuum; then it is the safety backup. You can prove that by stopping the engine, pumping the pedal three times to exhaust the stored vacuum, then you'll see the pedal is very high and hard. Also, when you have the master cylinder off, have a helper push the brake pedal and you'll see the push rod come out of the booster. That shows it is pushing on the piston inside the master cylinder. A booster that becomes defective while it's on the car will cause a high and hard pedal.

If people are trying to convince you the booster is the problem, remove one of the steel lines, then watch how much fluid comes out of the port when someone pushes the pedal. You can also screw in plugs into both ports, then you'll find the pedal will hardly move. Be careful doing that though because you'll risk damaging the threads or the seat the lines seals against.

It IS possible to have a problem with the booster causing a low pedal but only if something is assembled incorrectly, not once it's been working.

Aside from the obvious air in the line, a low pedal can also be caused by a leak, by rear drum shoes not properly adjusted out, and by a sloppy front wheel bearing. With a sloppy wheel bearing, it has to be so bad that it was making noise for quite a while. The clue is the brake pedal will be high and firm when the car is standing still but it will go close to the floor the first time it's pressed after driving a while, often on bumpy roads. The wobbling rotor pushes the piston back into the caliper. You have to push the pedal unusually far to push that piston back out before any fluid pressure will build up.

As for bench-bleeding, you can get away without doing that but then do-it-yourselfers think they have to bleed at all four wheels. That is a whole pile of air that has to be moved out, and that's more work than any mechanic wants to go through. Ten minutes of bench-bleeding can eliminate an hour of bleeding at the wheels and lots of fluid.

Another trick if you think there's still air in a front line is to use a screwdriver to pry a piston back into the caliper as far as it will go. That will push the brake fluid up into the reservoir and wash any air in that line up with it. You can even have someone watch in the reservoir to see if any air bubbles show up. It doesn't take much air to make the pedal go all the way to the floor.

By the way, when doing any pedal bleeding with a helper, never allow them to push the pedal more than half way to the floor because of that corrosion, but there is one exception. That's with your new (rebuilt) master cylinder. It takes a year or two for that corrosion to develop. Before that occurs it is okay to push the pedal all the way to the floor. That is similar to what you're doing when you bench-bleed it.

The only way to get a new master cylinder is through the dealer, and that is normally only done when the car is under warranty and the manufacturer wants their parts to be used since they're paying for the repairs. You will only find rebuilt units at the auto parts stores. You can also often find rebuild kits but they are not a good choice for multiple reasons. First of all, a professionally rebuilt master cylinder with a warranty is almost always less expensive than the rebuild kit. To do it yourself, it takes a lot of time, costs more, and if you mess it up you pay and do it again. Third, since the late '70s all master cylinders are made of aluminum to save weight. Since aluminum corrodes and forms aluminum oxide within seconds of being exposed to air, the bores are electrically coated with an anodized coating. We used a hone on an electric drill to clean the bores in older cast iron master cylinders, (and wheel cylinders), but with aluminum ones you can't use anything that will scratch that coating. Only brake cleaning chemicals and rags are acceptable.

When bleeding brake systems, do not be hasty. Slow and short strokes is all you need. Hasty bleeding can kill the mater cylinder more so if it is a used unit.