Why does my brake pedel go down after I replaced my master cylinder?

have someone have there foot on the brake & start vehicle, inspect the flexible brakelines to see if any are balooning out. You could also clamp off one flexible line at a time & press brake & when you find the brake at fault the pedal will go hard.

master cylinder shot . or you have lost all fluid start with checking master cylinder

are you loosing brake fluid? do you notice any leaks? it could be a bad master cylinder or a bad proportioning valve, master cylinder is more common failure.

Check on the upper part of the pedel to see if the clip that connects the pedel to the clutch master cylinder has poped off If the clutch pedel is on the floor this is a likely sulition and relitively easy repair you just reattach the clip

BY TRUCK HAVING ABS BRAKE.YOU NEED A SCANNER TO BLEED BRAKES THE SCANNER HAS TO RESET OPERATION OF THE BRAKE HYDRALIC SYTEM BEFORE YOU CAN BLEED SYSTEM.

You have to check that rear brake cylinders doe's not leak fluid. Check your master fluid reservoir when you pump the brakes.
You have to bleed air from the system, and check the vacuum hose at the master brake cylinder.

If you are not having to add any brake fluid to the master cylinder the master cylinder must be replaced.

I would first bleed the brakes again -- maybe there is still a small amount of air still in the lines. If this doesn't work and there is no visible leak, I would have to suspect the master cylinder is "leaking by" internally. A new one shouldn't be failing already, but it's happened before.

Let the fluid out of the master cylinder or from a line. If the pedel  then goes down w/o the fluid, then the booster is working ok. Rebuild or replace the master cylinder. 

refill the master cylinder with the proper break fluid then jst bleed them again and you should be good to go