The brakes work part of the time?

You're describing the common symptom of a failing brake master cylinder. The clue that there is not an external leak is the pedal is high and solid at times. If a rubber flex hose or some other part developed an external leak, you would never get a good pedal, and the master cylinder's reservoir would run empty.

First check the fluid level in the master cylinder's reservoir. This article may be helpful:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

If the fluid is very low, there may be air getting sucked into the hydraulic system. Add just a little fresh clean brake fluid from a sealed container. Press the brake pedal no more than halfway to the floor a few times to work any air out. If that solves the problem, the most likely cause is worn front brake pads. Once those are replaced, it will result in the fluid level rising up a lot in the reservoir. That's why you shouldn't add a lot at this point. Excess brake fluid can spill out creating a mess.

There are other potential causes for gradual loss of brake fluid. Look for wetness, especially around rear wheel cylinders. Brake fluid leaking there will often show up as wetness on the inner sidewall of a tire. With the possibility of worn front brakes or an external leak, this is the time for a brake system inspection by your mechanic.

If the fluid level is not low in the reservoir, the most likely cause of an intermittent low brake pedal is a failing master cylinder. This is real common after the brake pedal was pushed all the way to the floor during a recent bleeding procedure or when adjusting brake calipers after new pads were just installed. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Running the brake pedal to the floor runs the rubber lip seals over that crud and can rip them. That results in a slowly sinking pedal that often takes two or three days to show up. Even if there is some other external leak, the master cylinder may have been damaged too when you pushed the pedal to the floor, so be prepared for that.

If everything points to nothing more than a defective master cylinder, start by looking at this article:

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

If you want to do this repair yourself, let me know before you start. I have a trick that will let you avoid having to bleed at the wheels. This makes the job a lot easier. Also, if your car has anti-lock brakes, you can create a frustrating problem by trying to bleed at the wheels. Air will get forced down the lines and can pool in the hydraulic controller chambers. A scanner is usually needed to bleed that air out.

Let me know how you'd like to proceed.