1997 Freightliner Century Poor Breaks

Hello fellow drivers,

My Dad and I bought a 1997 Century with had excellent breaks when I first drove it.
After I took it for its first trip from south (warm weather) to NorthEast (below freezing) I started having issues with my breaks. Here is what happened:
I forgot to pull Red parking button out and disconnected the red glad hand. Air started coming out, so I pulled the red button out. Everything went back to normal. When I got back to the truck to drive, I noticed that breaks became very mushy and unresponsive - I had to press the break pedal very hard to get any breaks. Also, even though the air system was under normal full pressure, I had a buzzer going off and a signal that the air pressure was low and the parking break was still applied ( both yellow and red buttons IN).
Next day, I noticed that when bobtailing and I push the break pedal, air was coming out of the blue glad hand, so I changed the TPV, since it was leaking. That unfortunately did not fix the poor breaks problem.

Next step I changed the foot break valve (on the firewall), adjusted all breaks and changed worn out drive axle breaks - still did not solve poor breaking.
So, as of now, all breaks are adjusted, and good condition. New TPV and new foot break valve.
When driving and beginning to apply pressure on the break pedal, I feel that breaks (possibly front) catch a bit and then as if they "back off" and I have to press hard on the pedal to stop the truck.
Also, if I push the break pedal all the way to the "metal", it feels as if it "falls through" all the way to the floor (if that makes sense). I have not done anything to the slack adjusters (they seem to be pretty old). S-cams are not worn out.

Please help with advise.

Thank you!

 

I forgot to mention that I am not a professional mechanic and do not know the air break system component very well. in you advise, try to use simpler terms ))) Sorry. I am learning and will know everything after a while. Thank you all in advance.

 

Is it part of the foot break valve that is mounted on the fire wall? Is it internal to it or is it the external little item that bolts on to the foot break valve?

 

Got it. Well ,I changed it with a brand new one today...Hmmm

 

When I am towing a trailer and step on the break the only thing that I am able to distinguish is the fact that the truck breaks definitely lag - trailer engages first and the truck seems to start helping after that.The symptoms that I described earlier and hard to notice, because trailer breaks start working immediately. Other thing is when I am towing a trailer and start pressing on the break pedal with increasing pressure, breaks grab on pretty hard. best I could remember.

 

I noticed the break pedal "falling through" only today when i was without a trailer sitting at a parking lot and deciding to press the break pedal all the way down. I felt normal resistance of the pedal, but closed to the end of the pedal range, i felt and "empty pocket", pedal fell through, with some more resistance a bit before it reached the floor.