Brakes pump up but will not hold the pressure

Was this a used master cylinder? Are you losing break fluid?

Then there has to be air in the system yet. One thing to look for on trucks and minivans is a height-sensing proportioning valve by the rear axle. Those are used to adjust rear braking power on vehicles than can have a wide variety of loading conditions. If you have the vehicle supported on jack stands, with the axle hanging down, that is the same as being lightly-loaded. The proportioning valve will block fluid flow to the rear wheel cylinders. If the flow is real low, air can remain trapped in the line, especially where it goes up over the axle. You may need to disconnect the link to the proportioning valve, and move the lever on the valve to mimic a heavy load, then bleed the rear wheels. Setting the vehicle down on the wheels should work too.

Do not overlook something that causes the rear brake shoes to move too much. They can be badly out-of-adjustment, or a lining can be rusted off the frame, Two things that are often overlooked are the shoes toward the front must have the shorter linings and those to the rear have the longer linings, and a parking brake cable rusted in the partially-applied position will falsely make the shoes look like they are fully adjusted. To check for that, remove the drum, then look at the parking brake strut bar between the two shoes. You must be able to push that bar forward with thumb pressure about 1/8" against the anti-rattle spring pressure. If that bar is tight, the parking brake is not fully released. Also, be sure both shoe frames are resting tightly against the large anchor pin at the top of the backing plate.

Got your message about the proportioning valve. The normal valve is inside the combination valve. That sits on the frame rail right under the master cylinder. Follow the two steel lines from the master cylinder to it. That valve will not cause a low brake pedal.

When you have a height-sensing proportioning valve, that is in the rear. It also will not cause a low pedal on its own. It can cause difficulty in bleeding the air out of the rear system, and that is what can cause a low pedal.

Pop the cover off the master cylinder. Have a helper jab the brake pedal very quickly, about a quarter to halfway down. You should see two spurts of fluid squirt up in the middle of each half of the reservoir. If you do not, remove the master cylinder and inspect the push rod on the end of the booster. Some have a threaded shaft than could be out-of-adjustment. Check if that shaft moves forward when the helper pushes the pedal. That shaft is not connected directly to the brake pedal. When you push the pedal, it opens a valve that lets vacuum come in. That vacuum pulls on a rubber diaphragm that pulls on the shaft that runs the master cylinder. There is only a backup mechanical connection when there is no engine vacuum.

If you do get a nice pair of squirts, we are back to air. This may sound stupid, but more than once I have run into front calipers that were switched side-to-side, and the bleeder screws are on the bottom. Less common is a rubber flex hose that is expanding due to a torn outer reinforcement.

Why was the master cylinder replaced? Any chance the brake fluid was contaminated with a petroleum product like engine oil or power steering fluid?

One thing I overlooked, are the replacement rear wheel cylinders the same diameter as the originals? If they are larger, they will take more fluid resulting in a low pedal.

For what it s worth,

I am going share the technique I use. (The are other ways, I have done well this way)

Everything CARADIODOC shared is gospel, the only thing that might be overlooked or taken for granted is the actual 'way' you may or may not be doing the bleeding. Kind of like learning/ performing CPR by watching "ER" on TV vs. Taking a Red Cross forty hour hands on class.

When you finally get down to the actual bleeding in my link, kind of look at it in steps and in slow motion, unless you get the 'hands on" part right, the all of the air is not coming out!

Sometimes another approach might make things click. See if anything in here might aid you.

https://www.2carpros.com/questions/1998-ford-f-150-cant-get-brakes-bleed

Return with good news/ or more questions.

The Medic

Unfortunately the only way to now if the wheel cylinders are correct is to take an old one apart and read the size on one of the rubber lip seals. The sizes increase by sixteenths of an inch, but on the outside they all look and mount the same. This is something to keep in mind, but the more common symptom of a replacement that is larger than the originals is the rear wheels will lock up too easily under moderate braking pressure. Most of the time a low pedal won't be observed if the cylinders are just 1/16" larger.

Check for that spurt of fluid in the reservoir. If you do not get that, check the adjustment of the brake light switch to be sure it is not holding the pedal down a little. That can prevent the lip seals in the master cylinder from taking a new bite of fluid. Also, loosen the mounting nuts where the master cylinder bolts to the booster. Let the master cylinder push forward about 1/8". If you start to get a better pedal, the push rod in front of the booster is adjusted too long. That usually applies to import cars, but check it anyway. If it's a push rod length issue, you should get the fluid spurts now.

Double-check that this is the correct master cylinder. Chrysler has always been known for real good parts interchangeability between models and years, but that means different size parts all look the same. The cast iron master cylinder was used through the 1979 model year. There are two things to look at. The diameter of the pistons has to be the same as the original. Also, while there were not any vehicles back then that had rear disc brakes, it needs to be mentioned what the difference is. There is a "residual check valve" in any port that feeds drum brake wheel cylinders. That valves closes and blocks return fluid flow once the pressure drops to ten pounds. That pressure prevents the rubber lip seals from falling down, and it prevents air from entering through what is, in effect, a one-way check valve, when barometric pressure goes up. That ten pounds is not nearly enough to overcome the pressure of the brake shoe return springs, so it does not keep them applied, but if that valve got left off when the master cylinder was rebuilt or assembled, you will need to move the pedal enough to build that pressure at each pedal application. The more common symptom is the pedal is fine at first, then gradually sinks lower and lower over weeks or months, then, by the time it gets bad enough to warrant diagnosis, you repeatedly find air that bleeds from the rear wheels.

If nothing pans out by now, you might want to consider manufacturing one or two plugs to block a hydraulic circuit. If you install one of them in place of the steel line leaving the master cylinder for the rear brakes, (rear port), and you get a high, solid pedal, there is still air in the system or the shoes are moving too far. The threads are a special thread, so when I made these plugs in the past, I used an old fitting and filled it in by brazing the hole closed. Next, reinstall the steel line, then do the same thing at the outlet of the combination valve. If you still have a good pedal that way, loosen the connection where the steel line connects to the rubber flex hose at the rear axle. Loosen the end at the bottom of the body, not the two lines that leave the brass block bolted to the axle. That is the highest spot where air can get stuck. I have bled at places like that by myself a few times. I drive old stuff, including an 1980 Plymouth Volare and 1988 Grand Caravan, and I live in the middle of Wisconsin where they throw a pound of salt on an ounce of snow. My van is so rusty, the carpet is the only thing holding the front and rear together, so it is understandable I go through brake lines a lot.

Once you reassemble any lines near the master cylinder, it is not necessary to bleed at the wheels. Push the brake pedal slowly about half way down, then let it release quickly. That will push fluid down and air will float back up. When you release the pedal, the returning fluid will wash the air bubbles back into the reservoir.

If there is a little more air in the lines near the master cylinder, loosen the steel line nut, then have a helper push the pedal slowly. It should take about twenty seconds to push the pedal half way down. You will see air bubbles coming out. Tighten the nut, then holler to your helper to release the pedal quickly. Do the same thing a second time if necessary, then do that for the other line. This is my standard procedure when replacing a master cylinder. I never have to bleed at the wheels. If the pedal is pushed too fast, some air might sail right past the nut and go down the line.

I do not think this applies, but if your master cylinder sits real high in front, you may need to raise the rear of the vehicle, then slowly stroke the pedal to get the air stuck in the front of the cylinder to float into the reservoir. That used to be a big problem with GM trucks. I do not think Chrysler ever used such a miserable design, but it is worth mentioning.

I will be back tomorrow to see how you are doing.

Those spurts prove the master cylinder is working. When the pistons in it just start to move, the fluid being pushed finds it is easiest to flow up through the fluid return ports into the reservoir. Those are the spurts you see. Once the pistons move about 1/4", the rubber lip seals move past those ports, then the fluid ahead of them is trapped, and has no choice but to become pressurized as the brake pedal is moved further and further. The fluid has to go to the wheels. If it finds that easy to do, the brake pedal will go to the floor with little or no pressure being felt.

I know that sounds simplified, but it might lend itself to looking at the problem in a different light.

You might consider using a vacuum-operated bleeder at the wheels. The most common are a hand-operated pump with a collection cup. There is also a compressed air-driven pump that is typically used for evacuating AC systems. Either of those will cause any air in the system to expand and be easier to pull out.

One clue to look at is the fluid coming out will be clear if it has made it from the master cylinder. If it is dark, which is normal from being hot, the new stuff has not made it to the wheels yet, along with any air in it.

Also, be aware that when you do vacuum-bleeding, air is going to sneak through the threads of the bleeder screw and show up in the fluid. Those bubbles will be very small, and many of them in a steady stream. You can ignore them. Air coming out of the system will be very few large bubbles.

Another alternative is pressure-bleeding with a "bleeder ball". Those have gone by the wayside because today there are just too many variations of reservoir adapters. They can be messy too. Most of us get satisfactory results from gravity-bleeding.

When people resort to pedal-bleeding with a helper, it is easy to develop hundreds of pounds of fluid pressure. At that pressure, air will contract and it will dissolve into the fluid, only to show up later as a mushy pedal. Bleeder balls hold about four gallons of clean brake fluid, and compressed air, separated by a rubber bladder. A built-in regulator limits that air pressure to fifteen pounds. That is plenty to force the fluid through the system at a pretty good clip, but not enough to dissolve the air into it.

You might have to visit an Automotive program at a nearby community college to find a bleeder ball. We had one for training purposes, only because it could still be possible to find some at repair shops, and the students needed to know what to do with them.

I feel compelled to add another unrelated chapter to this sad story. When I was a student, a few years ago, we had an engine with an internal oil leak that caused a total loss of oil pressure. The only way we were able to find it was to pressurize the system with the engine taken apart, and we did that with the bleeder ball. (The cause of the leak became painfully apparent long before the four gallons of oil was used up. The horrendously-important detail though, was once we were done with the procedure, the instructor destroyed the bleeder ball.

The entire brake hydraulic system can be contaminated with a single drop of petroleum product, like engine oil, power steering fluid, transmission fluid, axle grease, or penetrating oil. The instructor didn't want to risk anyone using the bleeder ball for brake fluid in the future.

Later, when working at a mass merchandiser's Auto Shop in the 1980's, we learned about a problem they had in another state where an inexperienced mechanic used a clean rag to wipe out a funnel used for engine oil, then he used it to fill the brake bleeder ball. The minor residue in the funnel introduced enough contamination to destroy the hydraulic systems in over a dozen cars. The only proper repair is to remove every part that contains a rubber part that contacts the fluid, flush and dry the steel lines, then install all new parts. That gets to be real expensive. It's the combination valve, with its rubber o-rings, that gets overlooked. The contamination will leach out of that and recontaminate the new fluid and parts.

The reason I included this chapter was to point out one reason it is so hard to find bleeder balls today. I would not want to trust my cars to the previous person who used the tool, and possibly contaminated it.

Yup; I'd go with the vacuum-operated bleeder and I'd get rather aggressive with it. Remember to observe if the fluid coming out is clear or dark.