wheel cylinder
Another possibility I've run into before in cars that have sat for a while is a siezed wheel cylinder or caliper. Usually it happens as a result of whatever moisture was in the brake system settling into the wheel cylinders and corroding the aluminum pistons to the steel (or iron) cylinder walls. On a car that is driven a lot, or hard, the cycling of the brakes and the heat they generate keeps moisture woes at a minimum, and the brake fluid lubricates the metal parts, but on a car that has sat...
Unfortunatly there is very little short-term indication with out romoving the calipers and wheel cylinders and cycling each's piston(s) to make sure they are all free. You might not need to actually remove the rear wheel cylinders, you should be able to wiggle the shoes to verify the piston movement once you have the drum off. Don't try to use the brake pedal if you have a drum off or a caliper not on a disc, though, as the results are usually aweful.
If all of your driven cylinders check out, there is one more thing you, and every other Opel owner, owe it to you car to do: Flush the entire brake fluid system with fresh brake fluid. This will also acomplish the bleeding (and how!) and if you have never done it yourself, do you think anyone else has in the last 30+ years?
For anyone not familiar with the basics, I'll outline them below. If this is too long, Gary, then edit as necessary.
Buy a couple of the large containers of brake fluid.
Put at least 1/2" of brake fluid into the bottom of a fairly large container. (I usually use an old round wiper fluid bottle 'cause I have several, they stand up by them selves, and they have the child-proof lids.) This brake fluid doesn't need to be new, and it doesn't come out of you 2 large bottles. Use-up that little bottle you had in the garage way too long.
Starting with the wheel farthest from the master cylinder, crack the bleeder, put a 24" piece of vacuum hose (or similar sized tygon tubing so you can see the fluid) on it and submerge the other end into the brake fluid in the bottom of your chosen bottle. As long as the end stays submerged in the brake fluid it will act as a gravity check-valve, allowing only the fluid out of the wheel cylinder to come out and no air to go back in. This way you don't need a helper opening and closing the bleeder valve.
With long, slow strokes, pump 3 or 4 times on the brake pedal and check the level in the master cylinder. Don't let the master cylinder go dry! Pump enough brake fluid through to us up a little over half of a large bottle of brake fluid, and until what comes out of the wheel cylinder comes out clear.
Close the bleeder on that wheel, remove the hose (in that order, if possible) and move over to the other rear wheel. With the same rig and hook up, flush out the other wheel cylinder with the rest of that bottle of brake fluid. When done with the rear you should have one empty bottle of brake fluid and one partially full jug of nasty brake sludge.
Do the same thing to use-up the other bottle of brake fluid on the two front wheels, starting with the passenger side. When you are all done, carefully pour the used brake fluid into the empty brake fluid containers for proper disposal, leaving the 1/2" or so in the bottom of your new new low-tech one man brake bleeder system. Label it properly so the wife won't try to use it, and get another bottle of brake fluid for the garage.
Take the car for a drive and see how it works. Every single time I've flushed the brake system the improvement has been dramatic and immediate. Good luck!