The tech got ahead of himself when he told me the monovalve was bad. The shop foreman cleared the diagnosis up and told me that the hose to the monovalve needed replacement because it was "collapsing" as JackD put it, due to oil exposure -- causing too much pressure to build up at the connections and ballooning.
New hose (special order) will be put on and the diagnosis will continue after that, if not fixed.
Cap is new. Thermostat was replaced about 6 months ago, so this could be the culprit. To test, can it simply be removed from water pump housing and see if problem is resolved?
Thanks again for the responses..... gotta love this forum!
'91 300E, 208k miles