2011 radiator pressure.

My son and I drove our 240k mile Prius to California from NY. Made it to east side of LA and in traffic noticed some wierd shaking on engine start.
Went to Claremont Toyota and they were awesome. The tech explained that there are head gasket issues , usually around 150k. He’d never seen one go over 200k without the problem.
Car is extremely rusty (NY salt tax) so not worth putting $$$ into.
Anyway, car still runs fine, just loads up cylinder when hit and engine off. We took the cap off overflow- hoping to alleviate the pressure when shut off, but still pushes water in cylinder and creates rough start.
These were the first to not have a radiator cap- is there anything else to do to lessen pressure?