83 Isuzu P'up bleeding like stuck pig

Why is a radiator pressurized? I mean, it seems to do the job (at least in cool weather and if I keep my eye on it) with the cap off. So that one doesn't have to keep refilling it?

Same principle as pressure canner/cooker. Higher the pressure, higher temp the coolant boils at. Lets you use smaller radiator and less coolant and engine runs more efficient at higher stable temps. Early cars had like 160degree thermostat. Later they went to 180 thermostats, then 195 thermostats and I think some newer ones have over 200 degree thermostat. Old cars had just few pounds pressure, less than 7 pounds, most from 60s to 80s used 13 to 15 pounds pressure and some newer ones are like 18 to 20 pounds. The water pump circulates the coolant through the engine.

Thing is your water pump is on way out, right now eliminating pressure stops gushing, but bearings are now wearing at an increasing rate. Not long until bearings seize and thus fan wont turn or if it does, the shaft may leave the water pump and make a nice new hole through your radiator.

You may get by for a short time making short trips, but you are playing Russian roulette with life of your engine.

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"What would you do with a brain if you had one?" -Dorothy

"Well, then ignore what I have to say and go with what works for you." -Eliot Coleman

They used to "rod out" radiators, using a set of small diameter rods that were pushed through the tubes in the radiator core after radiator diassembled. This is old school and I havent seen modern radiator shop that does this. First its skilled labor intensive and labor is much higher priced than it used to be. Also remaining copper/brass radiators are thinner metal so dont like being poked with the cleaning rods. As I mention aluminum-plastic radiators are purely throw away.

Bigger truck radiators now are put in tank acid and "boiled out". Radiator has to be in pretty good shape to withstand this. As I mentioned some shops will custom make a radiator to your size and cooling needs. These arent cheap. Just depends on cost of labor I guess, but few can compete with China/Mexico/etc so you will end up buying a new radiator. Radiator shops will have higher markup, usually cheapest to buy on ebay or one of the big chain parts stores like Autozone.

As I mentioned I bought an all aluminum radiator, welded together for my Ranger project. From China of couse. I did lot of reading, still hard to believe but they claim it will cool a much larger engine than same size copper/brass radiator. Plus doesnt have weakness of combination plastic-aluminum radiators. I am still slogging away at chasing last little gremlins out of my Ranger. Going to re-insure it tomorrow so soon as I get driveshafts will be able to drive it and put it under load to see how radiator stands up.

__________________
"What would you do with a brain if you had one?" -Dorothy

"Well, then ignore what I have to say and go with what works for you." -Eliot Coleman

Ok, I see. I figured there must be a reason for it. A water pump couldn't be like an automotive appendix...

It is not that I'm not going to get it fixed, but you know how it is in the country -- there's no bus service and you still have to get around. I've located a water pump for $64 here. Now I just need to line up the mechanic.