master cylinder help on 1995 Chevrolet 1500 Pickup

i replaced the master and slave cylinder about 2 weeks ago iv been trying to bleed it right for along time i cant get it to work right i can get it to work if i pump it for about 1 minute be for i drive it then it will work for about 15 minutes why dose the pedal go soft after that long?, i took it to a shop and they told me i had the wrong master cylinder so i went to 3 other auto stores to try and find the right one but all of the stores gave me the same one is it bad if the master cylinder isnt straight?

1 answer
Maybe the m. cylinder you bought is defective and if so you could return it and purchase one from another store.