The brake peddal on my 64 chevy c10 pickup randomly goes from firm to soft several times thorugh out the day.
based on your very limited description, you probably have a failing master cylinder. Have it replaced.
You either have a failing MC as Shadow suggested or you have air in your system, creating erratic feel soft vs. hard.
Have the system looked at ASAP. If the MC is failing you could be on the verge of total brake loss.
…ouch, thanks for the heads up. Any idea of a ball park cost for a new on?
If it’s the MC and you do it yourself, around 50 bucks after buying brake fluid and returning the core.
If you get it done by a shop, several hundred.
(edit)
Of course, those prices are based on more modern cars/trucks. With your 64, it might cost you more depending on scarcity/market for the part.