I can't get any pressure to my rear brakes . I step on pedal and can still spin the back wheels . I've replaced the caliper and still same thing . Can somebody help ?

Have you bled the back brake,s? Or May be bad master cylinder.

U still have air in ur brake lines... YouTube how to bleed rear brakes..

When you changed the calipers to the rear brakes, (and why was it necessary to do so?) did you notice any fluid coming out of the lines when you disconnected them? If not, there is an equalizer valve that splits the fluid/ pressure that may be preventing fluid from flowing. Check this ONLY if you are not getting brake fluid from the lines when you attempt to bleed them. If your not sure your doing it correctly, go to a reputable brake shop to do it.

Also if your brake hoses are old, they can break down inside and clog, like a heart attack, so u need to check the union before the brake hoses and see if you can pump fluid out, if not you have a problem upstream as the good answers above suggest.

had this problem for a few years on my 1987 chevy truck . it's not the proportional valve you still have air in the rear line at the master cylinder . i bench bled it still no fluid out of rear brake line at the master cylinder. so put the master cylinder back on the truck and i had someone to pump the brakes and i broke the line for the rear brakes at the master cylinder until fluid started out kept doing this until i got a good flow . then i had someone to pump the brakes and i went the rear wheels and there was plenty of fluid then i went to rear

Just had this problem myself... I had to bleed the master cylinder with a self bleeder thingy first