yet another off book question re hose thread as unions at circulator in RV and best hose washer

so i'm installing pressure gauges on this hybrid half auto style half hydronic style RV heating system on either side of the circulator pump. Because it is at atmospheric or very low pressure cold, it could conceivably show a little bit of vacuum at the return until the system heats up. I'm installing isolation valves at the same time to provide for pump service and i'm thinking that brass hose connection could be a good union choice for this application.

indeed plenty of utility pumps use garden hose thread connections and obtain performance without performance sapping leak at the inlet. but it did get me to wondering about the quality/material of hose washer to use given the temperatures involved. operating limit is 180 and safety cutout is 195 and the suction is on the return so it's old school and you've got a lower temp there. I'm guessing the point of no pressure change is sort of the 'expansion tank' which is old school. no bladder or separation of air and coolant and the relief is a 7 lbs. radiator cap with an overflow tank just like automotive radiatior (although many of those are 15 lbs. these days.)

Obviously there are rubber and fiber materials that make good union seals in heating for the circulator flange o-rings and the G-unions. If i recall there is rubber and there is viton which is some kind of augmented rubber. Obviously many average hose washers in service connecting many hot water hoses to washing machines which see a buck and quarter but not really high hydronic temps. There are some folks, e.g. "backyard dudes" on amazon who advertise that their hose washer rubber compounds are good to 250 degrees. Many others don't offer a rating but if I knew what material to look for or what experience folks have had I can factor that in to the decision to spend an order of magnitude more for hose washers here.

thanks,

brian