There are a number of things that can cause that. The most common one is a defective master cylinder. Internal leakage allows the brake fluid to bypass the lip seal instead of getting pushed by them down to the wheels.
What's unusual is if it takes exactly three pumps to get good pressure every time. That can be caused by a problem with a rear drum brake lining. If the automatic adjuster mechanism isn't working and the shoes get out of adjustment, they will have to travel too far to contact the drums. That has to happen before any pressure can be built up. Normally that happens over a long period of time, as in years, but if it occurred suddenly, you might suspect one of the linings has rusted off the shoe frame. You should hear some grinding too when the brakes are applied. When you push the brake pedal, the shoes will move out part way freely, but when you release it, return spring pressure pulls them back very slowly. They are still part way out when you push the pedal again so this time they go out further. By the third pump the shoes hit the drums and you can build pressure. The clue here is you will not get a good pedal with three pumps if you wait an extra couple of seconds before each successive pump.
Regardless of the cause, if it isn't due to the master cylinder, it may be damaged anyway. By the time they get to be about one year old some crud and corrosion has formed in the lower halves of the two bores where the pistons don't normally travel. When you push the brake pedal over half way to the floor, the lip seals run over that crud and can be ripped. That causes the internal leakage I mentioned. The clue is the brake pedal will slowly sink to the floor when you hold steady pressure on it. That damage often takes two or three days to show up.
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Monday, March 15th, 2021 AT 12:47 PM
