Flat head engine are low compression -not sure of the exact number. The important thing is that the numbers you get are consistent. If 2- side by side cylinders are extremely low, the head gasket is gone.

As far as the 30% throttle issue, could be ignition or fuel. Describe the fuel system (I'm assuming propane), different types have different symptoms. Does it have a choke cable? The fuel line should go from the propane tank to some type of fuel shut off, with a wire or vacuum hose going to it. From there it should go to a regulator, either in the same housing as the thermostat in the upper radiator hose, or off to the side with 2 smaller hoses going to it. from there the hose should go to the carburator. Which of these systems does your truck have?

As far as the muck in the radiator, run some radiator cleaner through the cooling system & see what happens ( you may find a leak, you may not - sometimes folks use "stop leak" in the cooling system which will eventually turn to a muck at the radiator cap- but not always)

As far as the brakes, what are they doing? Your truck has a power booster & disc brake setup inside the rear housing assy. Not a complicated system, just an expensive & time consuming one to service.