I have no brake pedal in my 1999 jimmy i replaced both back calipers still no pedal

SOURCE: How do i properly bleed out my Jimmy?

I use a vacuum bleeder (abt $35) starting at the furthest brake line from the master cylinder & working my way back to the closest. If bled properly by keeping the maste cylinder full, this should solve your problem. Rick

SOURCE: Brake problem

air in the lines,bleed them,check rear brakes

SOURCE: 1999 GMC Jimmy Rear Brakes

where do you live meaning if you live on mountain or driving down steep incline every day you will ride brakes every day.that will wear on them a lot.use manual low1 gear coming down mountain.also buy a good brand brakes .i learned a long time ago if you pay more .it will last more.longer.

SOURCE: Brake pedal still goes to floor 2000 GMC Jimmy

It's very possible to be the master cylinder, the o-rings on the piston could be worn, letting the fluid seep back through instead of getting full pressure through the lines. Could also be the brake booster.
Try changing the master cylinder first, which is cheaper and easier, and see if there is a difference, you'll need to bleed the brakes again.
To check the booster,
With the engine off, pump the brakes until the pedal is hard, then hold pressure on the brake pedal and listen for air escaping, or the pedal goes slowly to the floor, Do that after the master cylinder has been changed, at least that will be eliminated.

Testimonial: "Hadn't thought about the O-rings on the rod thanks for the idea. Great Idea Thanks!"

SOURCE: 1997 GMC Sierra 5.7 changed master cylinder

check under your master cylinder,is there another clyinder below with one line in and one line out,if you do I found this is for the Abs to the rear,I had to replace this it was full of air,and did exactly what yours is now