88 s-10 brakes

i have an 88 s-10 with manual brakes - when i first push on the brake pedal it goes all the way to the floor - i can pump them and they will go back to normal for at least a couple of minutes then its back to the floor again - it doesnt appear that it is losing fluid so i dont think its a broken brake line - any ideas

could it be the master cylinder

any input is appreciated

i have an 88 s-10 with manual brakes - when i first push on the brake pedal it goes all the way to the floor - i can pump them and they will go back to normal for at least a couple of minutes then its back to the floor again - it doesnt appear that it is losing fluid so i dont think its a broken brake line - any ideas

could it be the master cylinder

any input is appreciated

If pedal drops slow under light pressure and no fluid leakage usually bad master cylinder.