just go easy, 3 wheel brakes should stop as good as 4 wheel brakes on a loaded bus, the steering will probably pull to the side with working brakes.
The only safe and cheap way to do the brakes on a 20 year old hydraulic brake system is to bite the bullit and replace every brakeline both steel and flexable, right from the start. the other option is to replace the leaaking line and bleed the system, then replace the next leaking line and bleed the system, and then..., by the time you are done you have replaced all the lines and wasted 2 gal of brake fluid countless hours of time chasing after parts and learnin to cuss like a sailor.
just some friendly advice from someone who has the t shirt
good luck