I have a 88 half ton 4x4 brake pedal goes down to abot 2 or 3 inchs from the floor master cyc and most of the breake lines are 2yrs old or newer, i get a full pedal if i pump the pedal once

You still have air in the system.(not bled properly) If you let the Master cylinder drop down to nothing then you need to bench bleed that before bleeding the lines again.

first check on every wheel make sure you see no wet . from brake cyclinders leaking. make sure brake fluid is full. when you replaced master cyc. did you bleed that before you hooked lines to it if not do that. abd bleed all brakes again. if still pedal goes down then it is the brake booster its self. hope this helps.

Sounds like a bad brake booster. To test, pump up the brakes with the engine off. While holding the brake pedal down,start the car. The brake pedal should drop a couple of inches and hold. If it doesn't, replace the booster.

Master cyclinders never look OK. You have to fill em full of brake fluild ( all the way to the top) and pump the breaks lots and hard. recheck the fluild and if it went down look under the truck to see where it's coming out. If you can't find it look in the floor under the break pedal where the pedal rod goes though the floor. Fluild coming out here is a bad master cyclinder, if no leak is found put in still more fluild and pump till you find wher it's runnung out. if the fliuld stays full and the pedal goes all the way down the master is bad. Masters fail alot on older Isuzu trucks.

You need to bleed your brakes, check for air in the lines making sure that you keep the brake reservoir full. Check for leaks from your master cylinder also.

Check your parking brake. if it goes down to the floor or does not hold, then your back brakes needs adjusted. bad master cylinders bleed off slowly to the floor while pushing on the brake pedal. If you need assistance on adjusting let me know.
Randy

You need to crawl under the truck and inspect each brake line for leaks. You also need to check wheel cylinders and calipers. The chances are pretty good that you will find a small leak in a line or a wheel cylinder leaking, pay close attention to the rear of the truck because that is normally where these problems occur. Hope this helps.

Did you bench bleed your master cylinder before you installed it? If not you may not be able to purge the air from it.

George

Sounds like your master cylinder is shot. Go to a parking lot, stop, and pump the pedal a bunch of times. Then get out and see if there is any fluid leaks by any of the wheels. That would indicate a brake line leak. My first guess, however, is your master cylinder.

change dump valve under master cylinder