Please, folks, let's get the mechanics of power brakes straight here. If the vacuum fails or the booster fails, the pedal will stay hard! Braking effort will be increased greatly, but when a booster fails, the pedal will not sink! If your pedal sinks to the floor, you are getting plenty of vacuum boost. It is more likely that one of three things is happening. Either there is still a problem with bleeding, and the boost is compressing the air in the lines more than you are able to do when unboosted, or there is a problem with the master cylinder, or somewhere a brake is vastly out of adjustment. If you've checked out your brakes and bled them, the master cylinder is the likeliest suspect.
There is no air, vacuum, or hydraulic gap in the mechanical connection between the brake pedal and the master cylinder. The booster is a design that fails safe. If it did not, then you would lose your brakes if the engine stalled.
When the engine is off, there is usually enough vacuum left in the booster for two or three pedal pushes. The pedal will harden up after that. If after leaving the car off for a while, you can push the pedal once and there is still some boost left, your booster is fine.
Here are a couple of things to try. First of all, pump the pedal with engine off until all vacuum is gone. Hold your foot on the pedal, pushing not too hard, but hard enough to hold the pedal down. Start the engine. If the pedal sinks, your booster is working. If it sinks to the floor, try pumping it a number of times (engine still on, boost still occurring). If the pedal rises and feels fairly firm and does not slowly sink down again, look for a very badly misadjusted or malfunctioning rear brake. If the pedal rises and does not sink down again, but feels springy or spongy, look for a bleeding problem. If the pedal does not rise, or sinks slowly under pressure, it's the master cylinder. It's common for a worn master cylinder to hold pretty well under very hard pressure, but to sink under relatively light pressure, because the rubber piston cups are designed to expand outward under pressure, so the harder you push, the tighter they contact the walls of the cylinder. Dual master cylinders are designed so that if one circuit fails the other will still hold, but the holding occurs only at the very bottom of the travel, so if you find some firm brakes, but only near the floor, that is probably the problem. You can test your brakes on loose gravel or dirt, and usually you'll find out right away which circuit is bad, or which wheel is not working.