Spongy Brakes Usually Boil Down to Fluid
Spongy brakes are generally caused by gas inside the hydraulic system. Remember that all passenger vehicle brake systems use a hydraulic fluid that transfers force through high-pressure brake lines.
If you get any kind of air or other gas inside those lines, then it compresses when you hit the brakes without effectively transferring force. That results in a spongy feel at the brake pedal.
The most common cause of this is water contamination of the brake fluid. Brake systems generate a lot of heat at the discs or drums during use, and some of that heat gets transferred into the brake calipers or brake cylinders and up the hydraulic lines. The heat can cause water to vaporize into a gas, resulting in a spongy brake.
Another possible source of gas in the lines is the brake fluid itself. It can actually boil if the brake system gets too hot. One cause of excess heat is a sticky caliper or wheel cylinder that causes the brake pads to continually drag on the disc or drum. Defective springs on rear drum brakes can also cause dragging. A dragging front brake pad will usually cause a car to pull left or right.
The first step in addressing spongy brakes is to change the fluid, which you have already done. It is unlikely that a vacuum booster, which is the major component that provides power assist in power brakes, would cause the condition you describe.
If the brakes are operating properly now and you have had them inspected for safety, it is best to delay further repairs until the condition returns. If it does, then you should immediately have a mechanic diagnose the problem. A second opinion may help.
In general, it is wise to have your brake fluid changed periodically, even though few manufacturers recommend it. Besides causing spongy brakes, water in the brake fluid causes corrosion that can ruin calipers and brake cylinders. Brake fluid changes are relatively cheap, so you should have it done every year or two.