Probs with rear brake pad change

[1992laser]

Hi Guys

while trying to change the rear brakes on a 1992 Laser i am unable to depress the brake cylinder all the way in. Therefore there is not enough room to fit over the disk (basically the brakes cannot open wide enough).

So is there any trick to depressing the brake cylinder? I have tried forcing it with a g-clamp but no luck. Any help will be appreciated

Cheers,
Grant