brake issuses

yes so let me start this off with first off i know im in teh wrong postin on the camaro forums but the gmc truck forums sucks balls and im reallly runing short on time to get it fix being tomorrows monday , well this is what h appened

i was coming home from work on friday , when i pushed down and felt some fuild squeeze by some seals is the best way i can describe it , i knew something was wrong and it wasnt holding presure , if i had constant presure on the pedal it would keep going down , soo i thought it was my master cyclinder after checking the drums and disks for any leaks that could be there or in the lines , but no leaks , okay fine i changed out my master cyclinder and bleed the brakes , now there is a possiblity that there is still some air inthe lines
but when im parked and not moving and apply presure to the pedal it depresses goes down and stays about where it should and might have a real tiny leak , but when im moving and i do it say going down the streeet at 45 , pedal depresses too much , its still not right ,
now when this all start i had a light come on , the parking brake light , some sensor for loss in presure or something?

but anyway i drive a 93 gmc seria 1/2 ton with abs

now my question to you guys is what else could be the problem i know there is like 3 vavles right under the cyclinder the control something dump and something else i cannot remember , sense im not 100% on what they exactly do i cant make an informed decision

is there anyway to pull codes off these trucks? i didnt notice the little port for it

i applogize ahead of time i just know there are some smart people inteh camaro forums
and was worth a try, id post in the correct section if it wasnt going to take a day or more for a response.. :( im gonig to try to bleed the brakes again thsi morning but i have to goto work on monday no choice , and would like to know my brakes are in good working condition =\

not only are you in the wrong vehicle forum, but this is also a problem.. hence our Problem Diagnosis forum!! :)

you can pull codes off of the truck the same way you would any 3rd gen camaro.. with a piece of wire.. I dont think it'l tell you anything

Check everything for leaks again.. It sounds like you've just got a lot of air in the lines.. After you installed the master cylinder, did you bleed it? Then bleed the four wheels, starting at the farthest tire from the master cylinder?
and, as your puling brake fluid out to bleed them, replace it with new stuff.. dont pour the old dirty fluid back in..

those valves under the Master cylinder are the check valves that regulate pressure between the front and rear brakes..

I would have to agree with damone it sounds like you have a shit load of air in the system,,,Start from scratch and bleed your master first with the hose kit that should have come with the master when you bought it,,If not then disconnect the lines and have someone pump the brake while you hold your fingures over the line holes,,let them pump slowley and let the air pass your fingures then hold tight over the holes and allow it to suck fluid from the master and let no air in passed your fingure tips do this a few times,,then do as Damone said start with the farthest wheel cylinder and pump and bleed making sure you dont run out of fluid in the master,,and leave the proportioning valve alone under the master ,,messing with this will cause you more problems.