if it's rear discs and the is handbrake incorporated into the calliper i'm 99% certain it's the handbrake mechanism causing the prob. it'll only be one calliper causing the problem but i always do both as sooner or later the other one will go so you might as well do it now while you have all the kit to hand.
basically the levers that attach to the handbrake cable have to return to the stop when the lever is released, if they don't then everytime you press the brake pedal the piston "adjusts" up a bit more, eventually it can't adjust anymore and is firm against the disc.
first, remove the cable and check that it's free to move, if not you'll need to replace them, only cheap from the likes of German and Swedish.
next step is to free the levers on the callipers, i find the easiest way is to remove the 10mm nut that holds the spring in place, whip the spring off and then, using plenty of lube work the leaver back and forth. if you push it right back to the stop you'll be able to lever it up out of the calliper body too to get more lube in there. don't worry about it coming out, it's held from within.
once you have BOTH levers working properly you need to reset the pistons in the calliper, these need to be pushed in and rotated clockwise at the same time to reset the handbrake adjuster, you can buy a special tool for this for about £20. once you have the pistons all the way back and the cables connected check that both cables have adequate slack in them by pushing the lever on one calliper to the stop then check the other lever is still on the stop. if pushing one lever to the stop pulls the other one off then the cable is too tight, i always leave plenty of slack in there anyway as often the limited travel on the cables can cause them to sieze, more is better.
once you're happy that the cables are good, the levers are both free and on the stops and the pistons have been correctly rotated to reset the handbrake then you need to drop the car back onto it's wheels (as you need the weight of the car to open the brake pressure limiting valve on the back axle) it's now just a simple matter of pumping the brake pedal anything between 3 and 30 times to reset the handbrake. the key thing here is to have enough slack in the cable to allow both levers to return to the stops.
hope that helps
PS: the reason no fluid comes out of the nipple is because you've jacked the car up by the bodywork allowing the rear axle to drop, this closes the brake pressure compensating valve and stops the fluid flowing
) hopefully car will be better soon!!
