Gas tank purge valve

Whatever you do the result will leave gas in the pipe, although at zero pressure but only if the heater doesn't have a safety device that cuts it if the pressure falls below a minimum.

Your propane tank is presumably sited in the open air? In that case the best option would be to turn off the supply and then disconnect the pipe and vent the gas to atmosphere.

If your pipes are soldered copper, they should be purged before attacking them with a blowlamp by disconnecting them at both ends and blowing air through them until the gas is gone.

When a car will not run right after refueling, it is usually the vent valve, that vents the tank.
If not opening, then using a small amount of gas will create a very strong vacuum, and not allow the car to run.
Try cracking the gas cap a little, to let air in and reduce the vacuum. Automotive Evaporative Emissions Part One

No the vent valve is at the tank . The purge valve on the intake manifold . The canaster stores the fumes from the tank. then at high enough engine speed / engine vacuum the purge is suppost open drawing the fumes in to the intake .
However its far from a perfict system look for leaks .

Maybe tank is full? Could be something was shoved in the tube that runs from the gas cap to the tank.

A loose gas cap or bad gas cap can cause this ,or filling gas tank while it running .or it could be a small leak in the evap system

I am not sure I understan tghe question. You should not have a lot of pressure in the tank. There are hoses that capture any vapors that are in the tank and eleviate pressure build up.
Most often I have found the engine light on because of gas tank vent, such as loose
or bad gas cap or the vent valve stuck open. The computer does a cycle ever so often
and maintains the vent pressure build up in the gas tank and vapor canister, and if the purge valve sticks open the computer gives a check engine light and code as the pressure is not in the vapor storage tank when called for.
If your worried about no pressure at all then I would change the gas cap and see if this helps

SOUND LIKE YOUR EVAPORATIVE EMISSION CONTROL NOT WORKING.SCAN VECHICLE FOR FOR FAULTY FUEL TANK PRESSURE SENSOR MALFUNCTION.AND THE CANISTER PURGE SOLENOID VALVE OR CANISTER VENT SOLENOID NOT WORKING.CAUSING EXCESSIVE FUEL PRESSURE AND FUMES BUILD UP IN FUEL TANK.CHECK CANISTER IT COULD BE GAS SOAKED NEED REPLACING OR VACUUM LINES GOING TO CANISTER PURGE SOLENOID OR VENT SOLENOID COULD BE BROKEN.PURGE VALVE LOCATED ON THE LEFT SIDE OF THE ENGINE ON A BRACKET AND VENT SOLENOID LOCATED ON A BRACKET NEXT TO FUEL TANK.CHECK BOTH MAKE SURE VACUUM LINES NOT BROKE OR DISCONNECTED.MAKE THE ELECTRICAL CONNECTOR INTACT ALSO.FUEL PRESSURE SENSOR ON TOP OF TANK ON THE FUEL PUMP MODULES.ALL YOU CAN DO IS REMOVE CANISTER SEE IF FULL OF GAS REPLACE IT.AND CHECK FOR BROKE AND DISCONNECTED VACUUM LINES.THE DEALERSHIP HAS TO HOOK UP DIAGNOSTIC EQUIPMENT CHECK IT OUT.BE CHEAPER BECAUSE ALL THEM EVAP PARTS NOT CHEAP THAT WHY LET DEALERSHIP SCAN IT SAVE MONEY IT WILL CUT THE COST.WARNING DONT KEEP DRIVING VECHICLE.GAS VAPORS PRESSURE BUILD UP ON HOT DAY FUELTANK WILL BLOW UP LIKE BALLON.PRESSURE KEEP BUILDING UP.

This could be caused by a couple of problems. First the regulator could be bad, or the tank is getting to hot. Which causes the relief valve to open and release pressure. This will happen when weather outside is hot.

there is a vapour purge line from the tank to a charcoal cannister. The line& cannister has been contaminated with liquid gas. When the engine attempts a normal purge from cannister it gets loaded with raw gas, causing rough running and cherck engine light. Replace cannister. Never over fill gas tank, especially on hot sunny days

have your evaporative purge valve solenoid check. because its controls the gas vapors going to canister purge or vent valve.take it to dealers have the evap system scan because its not venting or controling gas vapor.it suppose vent gas vapor to intake to be burn.if it dont work it causes excess gas vapor pressure built up in tank. make it hard to pump gas into it.