After installing the rear caliper with new brake lines i found that the pads won't fit. If i put one side the another one doesn't fit. I'm not sure the caliper piston is fully compressed.

Thank you all in advance

 

Really? Pretty obvious from 2nd pic, the answer is no.

Compress one piston, install pad. Compress opposing piston, install opposite pad.

What are you compressing the pistons with?

Oh, you did remove the MC cap and place a rag around it, yes?

 

I bought them remanufactured thus it's a brand new install, there isn't brake fluid on brake lines because they still disconnected, i tried to compress them by hand with the bleeder screw open, however was unable to do it then i ordered a quick pad spreader for a 4 piston caliper which will arrive sunday and decided to ask here first instead of damage the caliper.

 

Welcome- but your pics are so poor it's hard to tell much from them. The second shot appears to show the piston to your Brembo caliper is NOT fully retracted. The piston itself should be practically level with the caliper surface itself. Have you read the DIY section on changing brakes? It'll provide more in-depth instruction, but utilizing a breaker bar should help push it back into position. Be sure to bleed the brakes thoroughly to fill those new stainless lines.

 

So the pistons are dry. Got it. So, hook up the lines, pump up the brakes (either by foot or better, using a power/pressure bleeder such as a Motive.) Now, as you've discovered, trying to compress brake pistons by hand is futile; if this is happening after you receive your brake "spreader", retract one side at a time or however that tool works (never used one; instead.....

If you're doing it before you get the special brake tool, go get yourself a screw type C-clamp from Home Depot that is big enough to fit over each side of the caliper and brake pad backing, shove a suitable piece of solid scrap wood that fits (or cut to fit) to cover both pistons and tighten the clamp slowly until the piston surfaces are level with the caliper. Insert pad. Repeat to install the opposite pad. Again, remember to uncap the MC and put rags around it to catch back fill possibly spilling out.

Breaker bar, David? Hahaha, for some reason, my mind pictured a crow bar....

 

Yep- I use a small breaker (or crow) bar with an old sock or rag to keep it from marring the surface leveraged against to push the piston back in. I've also used a piece of scrap wood (with the old pad holding the piston in on the opposite side) to lever the piston back into place. But I've never pulled the brake line like the OP in this case. This is why I like the large pneumatic bleed tank from Griot's you saw me using at Sonoma to bleed my hot Brembos.