I was helping a buddy take down some rear calipers he got off a GL-10 tonight, when we realized that the pistons were stuck in place with corrosion and/or filth. Bummer. After some cursing and troubleshooting, we ended up pulling out as much of the top seal as we could, hosing down the piston with break-free, loading the caliper into a 14-inch C-clamp, and then compressing the piston to the bottom to break it free and deliver solvent to the cylinder.
Ok, so now what? We have crunchy, loosened pistons that still won't come out, and we don't have an air compressor. Well, we do have all my stuff for working on road bicycles...
We put the rubber brake lines back on the calipers, and removed the left over 10mm fitting from where the body-mounted hard brake line got severed. After cleaning up the cut end with a dremel, we got the small piece of flared metal tubing out of the 10mm fitting, and dropped the fitting in a small vice. In about 10 seconds, we drilled the hole through the fitting out to almost 1/4", so that it would barely fit a Presta valve from a road bike inner tube. We cut the Presta valve out, so that there was just a tiny rubber donut at the bottom, which sealed against the flared opening on the brake line leading into the caliper. We snugged everything down, threw the two-handed floor pump on that bicycle inner tube valve, and cranked it up to 130psi. A little prodding, and both stuck pistons shot out like cannonballs.
Now we just need to hone the cylinders, order new pistons, reseal the little demons, and its rear caliper conversion time.
On another interesting note, these calipers have the less common 30mm pistons, even though they came off a 4wd GL10.
I'm not sure there is any rhyme or reason that I understand behind the difference.