The simple things to check..
1. Check each caliper to see if its wet around the bleed nipple. You can do this without taking any wheels off. Maybe you're lucky and you just a bleed valve loosen up on you.
2. Follow the brake lines out of the caliper to where they become metal and check if any of the joints are wet. If you have SS lines than inspect the entire line for wear. If you have rubber lines, check to see nothing rubbed a hole in the line.
3. If everything is dry around the wheels, crawl undernearth and look above the rear differential at where the lines go forward. Check for spots of extreme rust that could have caused a pinhole leak. Lastly check the lines as they route through the engine bay
4. If everything is dry, replace the master cylinder. It's not the booster.
5. Oh, and if you are extremely extremely bad at general maintenance and let your pads get to the point where you're wearing down the backing plate, it's possible the piston on a caliper is so far pushed out that you over extended the seal. So if you have some pads that are absolutely down to nothing, than I would suspect that caliper.
Land Rovers are by far the best looking 4x4 on the back of a flatbed...