Brake issue. No leak, won't maintain pressure.

Im trying to troubleshoot a braking issue with a recently inherited truck. Im sure it was leaking but now I am not sure if the master cylinder is NFG. I have driven twice in the winter, the brake fluid was low I topped it up and made it 300km (180 miles) before the pedal got soft. Topped it up, drove it about 6km pedal went soft and I parked it. I just topped up the resevoir, pumped it till it built up pressure but it will not maintain the pressure nor am I able to see a drop in fluid level in the resevoir. I was never able to see where I was losing the brake fluid to begin with.... I don't quite know where to start. Should I just bite the bullet and change the master cylinder (I am sure it sitting empty through the winter was not the best thing to do)? Then look for leaks later? The truck is rusty..like Northern Ontario rusty... I was planning on fixing it up this summer brakes etc. to replace my super rusty Avalanche.. but the old Avy(365,000km) just broke a wheel hub (yes broke) and I am not sure where to throw my money atm.

 

If you aren't losing fluid then they need to be bled

Do a flush maybe?

If you keep topping it off it has to be going some where
If you don't trust the master then change it and flush any way.

 

The fluid could be leaking out at any of the wheel cylinders. Inside the drums etc. It also could be leaking back into the brake booster. Before you go throwing parts at it you really need to locate where it’s disappearing to.

 

I second what 15whtrd said, no visible leak, most likely going in the brake booster. Easy to check though, just loosen the MC and pull it back a little and it should tell the story.