losing air pressure when loaded

So a few weeks ago, I was coming up to a stop at about 5 mph when a car cut me off. I slammed on the the brakes to avoid hitting them. Ever since then, it feels like sometimes my brakes stick, but usually they will loosen up once I get going. Also since then when I pop my parking brakes, they hiss for about 6 seconds before engaging, when before it would be like a second max. BUT sometimes it wont hiss at all, just sort of pop out. Not sure what to make of that. BTW I'm in a 1990 379 Pete with a 3406b.

Now i'm not sure if thats related at all to what happened last night. but I randomly started to lose air pressure while I was loaded. I couldn't get it to go above like 90 psi. If I let off the gas for a second, I would immediately start losing pressure, and once I would get back on the gas it would start building again. It never went below like 65 psi. Once I was unloaded, (I haul an end dump) on my way back it built right back up to 120 and stayed there like normal. Another odd thing is that if I had the parking brake engaged, it made this faint very high pitched squeak. My flashlight was dead and it was dark so I couldn't quite pinpoint where it was coming from. Anyways I know this is all over the place, but if you guys might have any idea where to start looking I would greatly appreciate it! Thanks!

 

@NorthernMechanic Ill have to take a look at my air bags. It would make sense that it would lose air when loaded because of the extra pressure like you said. Thanks!