If it is your vacuum pump, you would get a braking effect, just without the vacuum assistance. So, in essence, what you would have is full braking effect, but only when you step really hard on the pedal. Also, if you suspect the vacuum pump, check your air filter housing and vacuum lines for oil.
Chances are, you have a fluid related problem. If you have replaced the master cylinder, then you need to check all other possible places for fluid leaks. Check each wheel's caliper and each flexible hose for evidence of brake fluid leaks. If you don't seem to be leaking brake fluid, then chances are, the previous comment was correct and you simply haven't bled them properly.
Bleeding them is quite simple. At each caliper, there is a bleed nipple which must be opened to let air out (put a piece of plastic tubing on the nipple and drain into a bucket or similar container) as pressure is built up in the system (get a friend to pump the pedal five times and hold while you release the nipple). Constantly check the fluid level, and do each caliper until all of the air is gone.
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