Do i need a new air dryer?

[Chamberpains]

That water and goop thats in your dryer doesn't all stop there when the dryer is that plugged up. Plus Bottles of airline antifreeze.... That's some seriously bad stuff to have run thru the system and be all over every valve in the truck. You probably need to go piece by piece and give a little TLC to everything in the air brake system. Airline alcohol dries the lubricant in brake valves, water corrods metals in brake valves, oils (engine oil) other than the ones that are supposed to be in the valves cause rubber seals to swell, Dirt and dust bind up moving parts... You can see why keeping dry clean air to the brake system is VERY important.

Yes, you can test the air pressure input to each brake chamber by hooking an air guage in at each brake relay valve output to the chambers. Its usually a 2 man job. 1 to operate the pedal and 1 to watch the gauge. It can be a 1 man job if you make your gauge with a long enough airline to go from the valve to the cab where you can press the pedal and watch the gauge at the same time. But you will get a little gauge lag because the air has to travel thru all that airline to gauge.

Hers a good video how the valves work and why its needed

https://youtu.be/f3LD8M0HKjo

You need as close to 120 psi out each side of the valves to get equal braking to each side. If its unequal or never gets up close to max air pressure the valve is not working properly. Relay valves are pretty cheap. Replace it.

Heres a good video of a guy pulling 1 apart and you can see the crap that gets in them.

https://youtu.be/IQLvDEnFx90

Now another part that is almost always overlooked because it is usually ridiculously hard to get to, is the treadle valve. But they very often go bad and or clog or corrod. Causing all kind of poor braking problems.
Heres a video (caution.. its a snoozefest but VERY good at showing what and how treadle valves work.) You'll see why dirt water and gunk can easily screw these up. The video is of a Bendix valve but almost all of them work very similar.

https://youtu.be/WTXtyxqvDtY

There's more ABS valves and other check valves that can go bad and need to be checked but these are the main culprits in braking failures if all other mechanical parts are in good working order.