Brake problems
 

I have replaced the entire brake system on my 1981 C3. Every time I bleed the brakes they work great for about 75 miles then the peddle starts getting soft. Does anyone have any ideas? I’m getting tired of having to bleed them every time I want to drive it. The only piece of the brake system that is not new is the actual pedal.

When you bleed the brakes are you actually getting air out?
It's possible that you may have a bad master cylinder. Even if it is new. These things happen.

Welcome to the forum Jim! You had few details on the Brake Job, Quality Parts? did you replace the Rotors? New fresh good quality brake fluid? How many miles on the brake job to date? Harley made a good point about the air in the calipers and which one has the most air. Do you think you have excessive rotor runout causing caliper pulsation, you can feel that in the brake pedal when braking hard. Are you sure you have the correct adjustment between the pedal linkage and master cylinder? You might want to look at the master cylinder closely if you run out of ideas. I`m just throwing mud at the fence to see what sticks! I did see an article about brake fluid.
https://cimg0.ibsrv.net/gimg/www.cor...50c9de0558.jpg

Welcome to the forum. If you replaced the rotors or removed and reinstalled the original ones without indexing them to how they were installed at the factory, then you need to check the rotor lateral run out. If it's out of spec, it can cause pumping of the pistons and cause them to suck air past the seals. So even if you bleed it successfully, within a few miles it will sucked air back in.

Did you bleed the master cylinder also?