brakes

95 gmc k-2500 that has brake problems The peddle is soft I have bled the brake put on new front hoses and replaced the master cylinderand wheel cylinders. New front pads and the shoes have only a couple of thousand miles on them. This truck has 250000 miles and had the abs light on for quite a while. I have been reading on this forum that you should not force the brake fluid back into the master cylinder. I have probably done this over 20 times in the last ten years with no problems. The booster seems ok, as it will drop slightly when the engine is started. After replacing the master cylinder when bleeding the brakes. After all the wheels were bled the front compartment on the master cylinder had dropped. I had topped it off after the rears where bled. Where did that fluid go? Any ideas on what else to do?
Thanks
Paul