It could be the master cylinder, BUT, odd that it was working before and now after the replacement parts it is not.
I would begin at the master cylinder and crack the line that goes to the front discs. Then have you depress the pedal slowly & steady. Put a rag under it to catch the fluid. If brake fluid seeps out, then probably not the master.
The fact that you said the pedal is "hard" would mean to me that pressure is building up, just not actuating your front discs. Generally when a master goes bad, the pedal will go to the floor.
If the master pushes fluid, then tighten the line back up and go to the line at the junction block and crack it open. Repeat the pedal push and see if any fluid seeps out. If so, then the junction block is probably OK. Keep doing this process right down to the hoses which attach to the calipers.
Now I have seen bleeders which are solid and not drilled with the hole that generally allows you to bleed the brakes through them. Verify that the bleeders do indeed have the holes drilled through to allow the system to bleed. If not, you will have to remove them to get the fluid to escape, and while you still have your foot to the floor, insert the bleeder screw and tighten up before you release the pedal.
Just go step at a time and see what turns up and this should identify where in the system the blockage/problem is occurring.
Does not sound to me like you have a mechanic who knows his stuff.