OK, stain down the booster can mean that the rear seal on the master cylinder is possibly sucking air - so one to watch.

What you're describing at the wheel cylinders sounds more like rust or something else has formed in the small hole at the end of the drilling for the bleed nipple. You might be lucky and get it to clear but I would personally just chuck a couple of new ones in. About a 20 minute job (once the drums are off) to replace each wheel cylinder. They'd be around $25 (or so) each to replace and, depending on the condition of the linings it might be just as easy to replace them too (that way the whole job is done for the next few years).

Have a real close look at the master cylinder and the condition of the fluid inside it. They sell for around $130 and are peace of mind and these days will last quite some time after replacement. Go easy on the fittings and you'll be right. You'll need to bench bleed the master cylinder before fitting it if you do replace it (which involves keeping it level in a vice, filling it front and back with fresh new brake fluid and slowly working the fluid into the working section of the cylinder with a phillips screwdriver (acting as the brake pedal plunger) while covering the fitting holes with your fingers. Messy but not difficult and don't get any brake fluid on any painted surface. If you do - DO NOT WIPE IT OFF! Use a hose or a watering can or similar to wash it off quickly, then wipe dry.

After fitting the rear wheel cylinders you'll then need to bleed ALL the brakes through properly. Make sure the master cylinder doesn't run dry while bleeding or you'll have to start again. Keep the lid loosely on top of the master cylinder (stops the fluid spurting back and all over the bonnet and windscreen etc), checking the level and topping-up every few bleeds.

I guess there is probably plenty of google info about it on the net....

Dave