Most likely it's the master cylinder, especially if the front brake work was just recently done.

With any master cylinder more than about a year old, you must never push the brake pedal more than half way to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Running the pedal all the way down, as in when people bleed the brakes with a helper pushing the pedal, or when they run the caliper pistons out after installing new pads, causes the lip seals run over that crud and can rip them. Often that internal leakage doesn't show up for a few days.

Check out this guide

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


There is another thing you have to watch out for that only applies to GM vehicles. You'll have to bench-bleed the new master cylinder before installing it on the car. If you allow one port to build a little pressure, a valve will trip and block two ports so no fluid will come out. We can discuss that further if it comes to that.

This sound help as well.


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

Please run down these guides and report back.

Thank-you will try that.That was very helpful. The new master fixed it the guides are great.