1998 Vauxhall Corsa Spongy pedal

I have a 1998 Vauxhall Corsa (B) with spongy brakes. The brake fuid is no older than 6 months, and it has been pressure bled 3 times in the last 3 months, by 2 different people. The brake pedal doesnt sink under pressure, which means I doubt the master cylinder is at fault. The brake pedal drops when I start the engine with my foot on the pedal, so the servo is OK. I think I have narrowed down the problem to the rear brakes. Yesterday, I had the rear brake drums (disks front, drums back) machined, to remove a lip that had built up on them. This seems to have healed the problem somewhat, but I can still get a firmer pedal by 2 methods: 1) When moving, forwards or backwards, applying the handbrake on hard for a second and then releasing it. Even after it is released, the pedal will be noticeable firmer. The same thing happens if I hit the footbrake hard when reversing. However, the firmer pedal only lasts a couple of minutes.

I've been told that my adjusters are fine (they are adjusting the rear shoes when I apply the handbrake, so they must be doing something). Would mis-shapen rear drums cause this issue? As maybe the shoes cant adjust properly if the drums are oval? I do also get a slight vibration when braking from high speed, and if I jack the rear of the car up and spin the wheels, the shoes only contact the drums for part of each revolution.

Any ideas? Many thanks