Help! Power drum brakes!

[jescates]

So after sitting 5 years, decided to get my 70 road worthy! So I changed out the drum shoes, wheel cylinders and rubber lines. I bled and adjusted everything! Brakes are still a little mushy, but I can live with that! The problem is, today I drove it for about 30 minutes, when I went to enter the driveway, I hit the brakes and it slowed, but when I hit the brakes the second time, the brakes faded to nothing! Had to use the emergency brakes to stop! So, any ideas what to change or adjust? I can pump the brakes a few times and the brakes work, but then it fades to nothing! With the engine running, I pulled the valve off the brake booster and air rushes out, so I think the booster is working. Can a master cylinder be the culprit? I checked the fluid and it's good, it's not leaking or anything! Getting a little frustrated right now! Thanks for any insight!