No brake pedal with engine running

There is an easy way to replace any master cylinder, except some that have four lines coming out, without having to waste time bleeding at the wheels.

Here is a guide

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

The most common thing we are running into now is if there was air in the lines and that got forced down to the wheels, if the car has anti-lock brakes, there will be a few chambers in the hydraulic controller that got filled with air. You will likely need a scanner that can access the ABS computer so it can be commanded to open some valves to let that air be expelled.

This usually does not apply to domestic vehicles, but consider that many power brake boosters have an adjustable push rod, and if it is adjusted too short, you'll need to push the brake pedal too far before the brakes start to apply.

The next thing I would be looking at is if the brake pedal is coming back all the way. Most need a return spring. If that is missing and only the springs in the master cylinder do that job, the pistons in it may not come back far enough to take a new bite of brake fluid. One potential clue is with the cover removed from the reservoir, you should see two nice spurts of brake fluid when you push the brake pedal rapidly. Unfortunately most reservoir designs of the last fifteen to twenty years does not really lend themselves to seeing that.

I never pedal-bleed with a helper. By gravity-bleeding, if there did get some air in the lines, it will usually float up and into the reservoir if you push the brake pedal slowly, so it takes about fifteen seconds to go half way to the floor. Never go past half way to avoid damaging the master cylinder, unless it is less than about a year old. Release the pedal quickly, then do that a few more times. Pushing the pedal slowly pushes brake fluid down the lines while letting air float back up. Releasing the pedal quickly washes any air bubbles into the reservoir along with the brake fluid that is rushing back.

There are always different piston diameters available for any given master cylinder design. The master cylinder has to be matched to the application and the diameter of the pistons in the wheel cylinders and calipers. If the diameter in the master cylinder is smaller than specified, the brake pedal will have to move further than normal to move the needed amount of brake fluid.

The biggest thing we do not want to find out about is someone contaminated the system with a petroleum product like engine oil, transmission fluid, power steering fluid, axle grease, or penetrating oil. Even one drop of those will cause serious damage to all the rubber parts that contact the brake fluid. That is a real expensive repair to replace all the rubber parts on the vehicle. Usually the symptom after a week or two is a brake that will not release and is smoking, and the brake pedal is unusually high and hard.

All of that relates to a low brake pedal right after work was done to the system. For a low pedal that occurred at some other time, the most common cause is rear drum brakes that are badly out of adjustment. That can be caused by rusted or broken self-adjuster parts, or a parking brake cable that is rusted tight in the partially-applied position. Next would probably be a caliper that is sticking on its mounting bracket and cannot slide freely to self-adjust. That was a real common problem on older Ford trucks. The least common cause, but worth mentioning, is the outer casing is deteriorated on a rubber flex hose, the reinforcing weaving has come apart, and the inner hose is expanding from the fluid pressure. That will not last long like that before the hose pops.

I have also run into calipers on the wrong side of the vehicle. Many use the same casting for both sides, then only the location of the bleeder screw is different. When you need a right caliper, but all that is available right now is a left, you can use it, but that puts the bleeder screw on the bottom where it cannot do what it is supposed to do. Those can be bled while holding it removed from its mount and held with the bleeder screw up. If the bleeder screw is snapped off or rusted tight, you can also hold the caliper with the hose up, and loosen the hose connection to bleed that line.

This is what you can do to see if the problem is at the wheels or the master, take three vise grips and gently close off the flex hose at each wheel. The pedal should be firm, if so recheck the rear brake show adjustment and see if the calipers slides are no seized.

Please let us know what happens.

Cheers, Ken

I think we've covered everything. I only added all the possible causes mainly for the benefit of others who may read this wondrous story while researching a problem. But it could be the power brake booster internal vacuum valve has gone bad.