Thanks for the information. Testing showed the valve hadn't tripped,so I did an experiment;I drove the truck up to about 30 mph and jumped on the brakes a couple of times.

Apparently,something was stuck,and this jarred it loose,now the brakes work fine. I guess I lucked out! I've been driving the truck every chance I get,working out some problems and burning up what gas is left in the right tank so I can drop it and replace the sender. Other than that and the excessively slow wipers' speed,the truck's about as done as it's going to be for a while.
Thanks again for the information on the brakes though;now that I have
two of these trucks,odds are that one or the other will eventually put me through the wringer on brake troubles.

Speed