brake piston stuck caliper

While putting the finishing touches on what I consider a nice 1978 XS400 build, the front brake began giving me trouble and the piston is now firmly seized in the caliper assembly. The master cylinder is not the proximate issue (new HEL hoses and master cylinder rebuild kit), the piston is just...

After viewing threads... Getting the old one out was easy. Now ..with new seals, new piston , everything lubed with brake fluid, the dam thing will only go in 7/8 " and stop. Any suggestions like heat or a shop vac to suck that thing in? I was told force beyond hand preasure should not be...