92 Dakata with poor breaks

I have a 92 Dakata with 200k miles on it. Bought new in 92. Replaced front pads ~20K ago. It doesn't get used much now that I commute 100 miles a day in a 2004 Toyota. The truck only sees light hauling for the pass year. The brakes are failing and I believe its the master cylinder because the padel slowly sinks towards the floor. I tried bleeding the brakes. There was a small amount of air in the rear left. Do youo think its the master cylinder and how much work is it to replace the master cylinder?

it's unusual for brakes to have air in the system unless you had a leak somewhere and the fluid all leaked out. Check the level in the master cylinder and see if it's full or low. Also check around the front calipers and the rear wheel cylinders for wetness. Try that first before tackling on the master cylinder.

Are you losing any fluid? Sometimes a bad M/C will leak fluid from the backside. Check the rear brake adjustment. If that's not right your brake pedal can go down more the first time you pump it but usually goes back up to normal on the second pump.