I have very little brake pressure in my pedal after days of bleeding and replacing the master cyl, rotor, and caliper. The rear drums seem to be in good adjustment. I see no leaks and when the engine is off i get solid pressure build up in the pedal. Engine goes on the pedal travels almost to the floor but builds up pressure if I pump it. The brakes do work but not the way they're suppose to.

You describe a brake problem of a master cylinder leak.
But the vacuum booster could interfere with the application of brake pressure on the master cylinder.
For a firm brake pedal and holding a leak inside the brake booster.
and rotor, front pad, drums, and brake band have to be in very good condition.

Hope it helps.

what is the year ,make and model.? does it have abs brakes

has to be air,try bleeding without the brake pedal,just open
the bleeders and let gravity run the fluid out,takes awhile but works.
(master cyl.cover off )

to bleed brakes is a relatively easy job for two people although it can be done by one without a pressure bleeder.NOTE keep brake fluid levels above minimum at all times during bleeding ( forgot to do it my self on more than one occasion and had to start over, trying to do too much at once) To do by your self you will need the correct ring spanner for bleed nipple, a clear tube that is a tight fit on bleed nipple ( long enough to reach down to floor) and a container with about an inch of brake fluid in it (glass jar will do it, is easier to see fluid level), Starting with the cylinder/caliper at rear furtherts from master cyl (longest brake line from tee piece) place spanner on bleeder then fit tube, placing other end of tube into container below level of fluid,. release master cyl cap/lid (no need to remove completely) undo bleeder half a turn, watch for fluid in clear tube. this should start to flow within 10 to 15 seconds (if bleeder isn't blocked with road grime clean if required), if fluid doesn't flow by it self (it should but) tighten bleeder then release again quarter of a turn, apply foot pressure to brake pedal, hold pressure on pedal till you feel pedal dropping towards the floor (half inch of pedal travel should be enough) ( fluid in container will stop air being drawn into brake lines when you release brake pedal) check clear tube for fluid/bubbles, open bleeder further quarter turn, if fluid has filled clear tube allow fluid to fill container a further eight of an inch, then close off bleeder (make sure no bubbles appear in tube whilst this is happening other wise drain more fluid till only brake fluid is visible in tube ) repeat process on opposite wheel cyl/caliper, then repeat process on front caliper furthest from master cyl , then the caliper closest to master cyl last. If two people are doing the bleeding the same clear tube and container are used,as is the order in which cyl/calipers are bled. One sits in the car and pumps the pedal until it has some resistance under foot (Whilst the pedal is held down) second person releases bleeder, first person keeps pressure on pedal whilst bleeder is released (pedal will travel towards the floor) when pedal has been depressed as far as it will travel second person closes bleeder before, the first person releases pedal and pumps the pedal again repeating the process until no air bubbles appear in clear tube at this cyl/caliper before moving to next cyl/caliper. good communication must be maintained to make this process hassle free

yes may need to bleed lines at master cylnder

Replacing brake pads involves pushing the pistons all the way back into the the caliper. This will force fluid up into the master cylinder. Sometimes it also allows some air to get past the piston seal and cause 'soft' pedal. In this case bleeding is required. Anyway the brake fliud should be completely changed by pressure bleeding at this time.

SOUNDS LIKE MASTER CYLINDER SEALS ARE GONE. REPLACE MASTER CYLINDER WITH NEW OR REBUILT UNIT.

Try having the dealer flush and bleed your system. It is very hard to bleed ABS systems yourself and have safe brakes that still work afterward. Bleeding non-ABS brakes yourself is easy not the same for ABS brakes. Valving, sensors and what-not require a tech and the correct equipment in my opinion. You do it wrong and you could ruin your ABS system. Do that and see if they firm up. I replaced my brake shoes/pads at the same time all new everything in back, drums/springs everything and new rotors up front. then I had the chevy dealer flush, refill and bleed system. Stiffer pedal and brakes work better. Keep in mind the brakes on 99 Tahoes are inaedequate, require new rotors often, heat up and fade/glaze pads regularly. I replace my pads long before they wear down because they glaze up and start fading early. I'll rough em up once maybe, next time, new ones. Every two brake jobs, new rotors for me. Just how it is. They will stiffen a bit and work better but they will never be awesome brakes. Just how it is on 99 and earlier Tahoes. Hope it helps. Very important to bleed correctly though. I'll bleed my 83 Toyota 4x4 myself but not the Tahoe.

always bleed front driver side first, front right,left back
right back.
make sure you always have oil in the master

Try a clean and adjust on the rear shoes, because sometimes when they aren't properly adjusted they won't seem to work properly. Also there is a common "squishy" petal feeling after brake work on most all larger GM's around that year, wich I have run into several times. Try test driving it, and put a couple miles on it, That usualy helps.
Good luck!