SC520, Piston Rings, & Bucket Pumps

One of my bucket pumps failed yesterday. Its the one I use my the SC520 cutting additive I used with water for most things. Today I decided to
open it up and see if I could do anything about it. It has was appears to be a brass or copper piston rig that seemed to float back and forth between two ridges on the piston. It had popped off the top side of the piston and the ridges that kept it on were pretty worn. I turned a quick groove on the piston and popped in a big fat o-ring. A tight fit at the neck turned out to be a perfect fit down in the range were it does most of its moving. The pump is working again. Now the question is how long will that plane old cheap buna rubber o-ring last down there? Would a buna-N or a viton ring hold up better?
I guess I could have made a new piston, and reused the brass/copper ring, but I had a machine running short of coolant and I didn't have any premixed to pour into the tank. The rubber o-ring got it going quick. The coolant was only just starting to foam a little. It foams up some when the pump starts sucking a little air.
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