I have 5 front calipers on my bench, which I soaked in WD40 through their LHM holes and than blown about 8 bar pressure into, but no movement of their pistons...

This worked a treat on 3 other calipers. I need one more, but no luck so far.

Anybody got a trick to get them moving without destruction?

Soaking in Diesel?

Many thanks, Bart

Usually air - but you've tried this, maybe hydraulic pressure? I.e mock up a pedal and cylinder and pump them out.

yup, that's what I meanwhile thought too. If no trick helps I'll just fit a caliper on my 2cv without pads and press the pedal. But that's for next spannering weekend as I am off to Cologne tonight.

try to push them in a bit to loosen things up, then blow them out with air, but preferably with hydraulic pressure, because it's much safer than with air.

air is risky ok you cover the piston with a rag etc but you usually end up with a face full of LHM at some point. if its really stuck either connect the caliper half to the master cylinder or fit an M8 grease nipple and use a grease gun you have to use a clamp and a bit of rubber to blank off the hole between the calipers.

I forgot to tell that I "gently" hammered in 2 pistons, but that didn't help that caliper. I like the greas gun idea!

I had the halfs pressed on a cloth on the bench, such that the bore to connect the halfs is more or less blocked and the piston won't fly around the workshop.

I pumped mine out with a hydraulic hand pump, There's pics of the caliper rebuild in my build thread.
Soaking the seals might make them swell making them even tighter. You could try heating the caliper with a heat gun to expand it slightly then the 8 bar air pressure.

You may plug the caliper half with a screw clamp and a handbrake pad
so that the open end is close, before you apply pressure, preferably hydraulic.

If that fails you can drill a hole in the presumably seized piston,
thread the hole with a 10mm tap and remove it by screwing in.

Thanks for all tips chaps! the grease gun worked a treat