Terrible service, bought a pre owned 2012 Silverad...

Terrible service, bought a pre owned 2012 Silverado 1500 6 months ago, 3 months ago I had brought my truck in since the brakes felt spongy and had to pump them in order to get them to work properly. Let the sales manager Corey Risling take it for a spin to see for himself what I was talking about. Said "there's nothing wrong with your truck" even tho I've talked with many of my friends that are mechanics and all of them agreed it's most likely air in my brake line. Corey said even for them to check it out and see what the problem was it would cost $300 JUST to check it out. Why risk my safety especially with winter driving? I definitely won't be dealing with them EVER again if they're disputing a simple issue as air in brake lines.