Muddy looking brake fluid

[HBlue]

My 76 C10 brakes went out Friday. The pedal goes all the way to the floor with very little stopping power.

I pulled the cover off and found the fluid a little low, and it has a muddy look to it. I had the calipers, brake shoes and one brake line replaced a couple of months ago.

Do I need to replace the master cylinder or power booster or both?

Should I bleed out all of the old fluid and replace with new?

This truck was driven only about 1000 miles over the last 4 years. I just bought it in January.