Think master cylinder is going out, had no breaks added fluid and pumped breaks seemed fine when to drive and went out . How can I be sure.

I do not have a shop service manual ... but you may have braking challenges in general if this 1968 still has drums up front. This car was notorious for not having good braking (if all 4 wheels had drum brakes).
Anyway ... was the brake fluid real low? ... below the minimum add fluid line? If it was still in the safe zone before you added any more fluid ... then you could have several issues:

• contaminated brake fluid
• hole in brake line
• master cylinder issues

If the fluid was below the minimum .... then there is probably a leak somewhere ... and should be inspected
Does the parking brake still work? That may help you troubleshoot.

if master cylinder is bad. the pedal will go to floor running or not.when the pedal goes down at a standing stop it is usually the master cylinder thats bad. and losing vacuum. thats if there are no other leaks at wheel cylinders etc. if putting on a new master cylinder, do not forget to bench bleed it! heres how. put the master cylinder in the vise, fill it full of fluid then pump the plunger with a dowel or something soft as to not ruin plunger part.do not let it run dry ! when fluid comes out from master cylinder and no more air is in it. fill it up and put on car. heres what I do! remove the lines from old cylinder on car. as long as you do not lose any fluid doing this, there is no need to bleed air from brakes when done. after on car make sure master cylinder is full of fluid. start the car and pump the brakes a few times, then you should be all set.

does the truck have rear drum brakes on the rear? if so then the fluid is leaking out of the rear wheel cylinders. or the master cylinder is leaking fluid into the brake booster.

WHEN FINISH REPLACING BRAKE PADS.MAKE SURE MASTER CYLINDER COVER ON FLUID LEVEL OKAY.YOU PUMP BRAKE PEDAL UNTIL BRAKE PEDAL GET FIRM HARD.IF BRAKE PEDAL KEEP GOING TO THE FLOOR YOU HAVE AIR IN THE HYDRALIC BRAKE SYSTEM.BRAKE SYSTEM NEED BLEEDING.DONT DRIVE VECHICLE IF BRAKE PEDAL FEEL SPONGY OR GO TO FLOOR.

If there is no leaking brake fluid then more than likley your brake master cylinder has failed.

your problem could be the Master Cylinder,first check the wheel cylinders for leaks if there are no leaks,then bleed the brakes.if that dosen't work replace the Master Cylinder there are steps to bleeding the Master Cylinder instructions come with it.

Visiually check the rear breaks and if it is not leaking you the need master cylinder replace. When replacing a master cylinder don't forget to bleed for trap air also.

If you want to change break master cylinder,first remove two break pipes then remove four nuts which is holding master cylinder to breake booster.careful breake fluid from the car body and your self.

put the new breake master cylinder tight four nuts, before you fit the braeke pipes get some body who can hold breake bedel down put the breake fluide tell him to pump breake bedel when the brake master take fluid hold your fingers where you want to fit brake pipes when you feel more pressure which is pushing your finger tell him to hold the bedel down while you tighting the pipes.
you dont need to bleade the wheels. it is ready.
the ather thing when you remove the breake master put the new one then fit the brake pipes fil with the breake fluide then bump so you must bleade all wheels

You most likely have had the Brake master hydraulic cylinder go out, if u don't know cars this is a job for a shop to repair.

Air in the lines, or a leakey line or caliper, do you see any fluid leaking on the rims? or under the car? if not they prolly just need to be bled.