That's because you're NOT really bleeding them. HOW are you bleeding them? If you have a leak, or air *trapped* in the system, you MUST pressure bleed. You should pressure bleed anyways.
You *may* be able to bleed with the 2-man method. When you say hard/from pedal. Is that true IF you let the car SIT for 10min?
If you pump the brakes, you compress the air in the lines. Then, after a while, the built up pressure bleeds back into the MC and you once again have air.
Or, if you have a leak. You may not feel it without the power booster. There isn't much leverage using a power booster without vacuum.
If you want to get it done quickly, then the 2-man method is the way to go. If you have time, get a pressure bleeder, and CUSTOMIZE A CAP. With a pressure bleeder, you can see "most" leaks if you leave pressure in the system for ~10 mins and look around. Also, with a pressure bleeder, bleeding is BORING. Just like changing oil. You do it, it *works, no BS worrying about ALL of the problems that *every* other method has. SNORE!!
BTW: MANY MANY MANY people have problems bleeding brakes. The reason is because there are a number of companies that sell SCAM and BS products that MAY work in SOME cases. Professional pressure bleed. It's the only approved method by Ford, GM, and most other companies. So, your (and many other people's) problems with bleeding brakes is due to BS advertising of BS products.
Hey, I got screwed *many* times buying BS products when I was a teen and learning about cars. (And, uhm, a few times since then also.
). I try to tell the people the *right* way to do things. Most of the time, they don't listen. But, enough do.
Also, with a pressure bleeder, you can bleed the MC. Uhm, do it CORRECTLY, or you'll have paint damage. To pressure bleed a MC.
1) Hook up the Pressure bleeder.
2) Get a junky FULL SIZE bath towel.
3) FULLY cover the fitting with the towel. Use 2 or more if you have to. I also like to cover the fender and cowl to be safe.
4) Loosen the fitting a LITTLE at a time. Do this UNDER the towel.
5) As you *feel* the fluid to escape, stop. Allow the fluid to drip *slowly* for 20-60 secs.
6) Do the other fitting.
7) Repeat #5 & #6 at LEAST once more. 2-3 time more is better.
8) Now bleed the brakes. I also like to do one round. Then do a second round.
9) If you have *any* air in your system after that, you have a leak.
I bleed 3+ of my cars every year. Never had a problem with my pressure bleeder. Well, except when I used the universal adapter and it got loose and sprayed brake fluid all over the place. A trip to the junk yard, a $5 MC cap, about 20 mins of work, and I never had to worry about making that mistake again!
For information on Bleeding Brakes, see:
Note: It has some out-dated info and links. Hey, I did that page ~?10? years ago.