If its going right straight to the floor-check the master cylinder could be leaking internally: https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

thanks for your information,with reguards to my brakes,but,i change the master cylinder and iam still getting the same problem,the brakes is very low.

Rear brakes are they drum if so open it up and inspect for wear and tear if any replace appropriate component and then adjust brakes shoes-