Look at your surge brake fluid. If it does not look like it does in the can it has absorbed moisture. It it looks like the new stuff then leave it alone. If you need to change it first find something like a turkey baster and suck out all of the fluid that you can and then fill the cylinder up to normal level before starting. It has been too many years since I had surge brakes and I don't remember the system design well enough to walk you thru from this point without seeing the trailer.
Brake fluid has an affinity for moisture. Fluid flushing depends on where you live and how damp/dry it is. Brake fluid in plastic bottles stored in a damp environment will absorb moisture and not be useable. Brake fluid stored in cans will have an almost indefinite shelf life. If you look you can still find fluid in metal cans. That is all that I buy.
I learned these things dealing with race cars over my lifetime and following the race crowd experiences. I also lived in a very damp Southern climate at pone time and ended up replacing brake wheel cylinders because I let the fluid absorb moisture and rust formed in the cylinders and pitted them. No brake honing doesn't usually repair the cylinder.
I am 79 now and still servicing my skid steer and tractors on my ranch. Now I live in the plains of Colorado and I look at my brake fluids when I have the hoods open on my vehicles and if they have changed color I will flush them.
Good luck
"YEARS OF LEARNING THE HARD WAY"
