I'm getting better at my brake knowladge but still have a lot to learn.My question is.I had to replace both of my rear calipers because one of them was messed up so you couldn't get the inside bleeder to work.Now about every two months I need to bleed the brakes again.I'm not losing any fluid but how can that be.If air can get into the system wouldn't you think that fluid could come out of the same place?

Did you check the runout of the rotors?

What dwright said is the most likely culprits. What you also have to check is something that happened to my '78...one caliper seal didn't properly seal. Instead of leaking fluid, it sucked air in every time I used the brakes. It didn't take too many miles before it showed itself and needed bleeding. Eventually the probelm was discovered and fixed.

It's not always the obvious, but check the obvious first.

If the rotors are indeed "pumping", as Dave says, there wouldn't be any fluid leakage out of the system. The warped/out of parallel rotors cause the pistons to pump in & out, due to the springs behind the caliper pistons, and this sucks air into the calipers. The design of the seals on the pistons will allow this sucking action, but are not designed to let fluid out the other direction.
Eventually, if you just kept driving without re-bleeding the system, you would have enough air pumped into it to actually apply the brakes on their own, or it would blow a seal on the master cyl.
That being said, be sure to check that there is no fluid loss from the master cyl. into the brake booster. If this is leaking, you wouldn't be able to see the fluid anywhere on the outside. If you take the two nuts holding the master to the booster off, and slide the booster forward, you will be able to tell if the master is leaking there.

Sorry, It sounds like the Calipers are bad. Mine were doing the same thing. I could bleed all the air out, Run the car about a week, and then I had to bleed the system again.

My MC/Booster are new.It don't feel like a warped rotor.I'll look over all you have said.The rear calipers are SS the fronts are orginal to the car.

It only needs to be a few thousandths over spec to cause the calipers to "pump". You won't necessarily feel the brake pedal pulsing. The first step in solving this is checking the runout on the rotors. Putting on new calipers will not solve the problem if the runout is excessive.
Scott

Make sure you don't have a bad wheel bearing, that will move in and out and do the same thing as a rotor with too much runout.

Even a new rotor can cause the same symptoms. If the surface of the hub that the rotor sits on is rusty/dirty, it will still not be parallel. Plus, I've gotten new rotors out of the box that were crookeder than snot. I've always turned new rotors, just to be sure they were true...and some piece of mind for me.
Also, it matters not how good/new the calipers are...if the rotors are fluctuating, they(the caliper pistons) will pump air. The only ones that do not are the o-ring type, and they will if there is enough run-out on the rotors, but it would have to be really bad.