Blowing water pump bypass hoses

[Bill Spain]

I just brought home my 302 powered Cobra. It was a 95 degree day and the coolant temp was below 200 for 3 1/2 hrs, i.e. no problems. The next day I went for a short ride and when I stopped and shut off the engine, the rubber hose cap that block off the water pump connection that typically goes to the heater blew apart and dumped my coolant. I bought a new bypass hose cap and put it on. I took a 30 minute ride with the coolant temp at about 180. When I stopped the new hose cap blew apart. Tried a new one and the same thing. The temp is OK until I stop and turn off the engine, then it heat up and builds enough pressure to blow apart the hose.

Any suggestion on what to check first.

Thanks for your input.