Back to your original question. The regulator will function ok without a cap however it is not a good idea due to rain, bugs road crap etc put some tape over it. Also current code requires all permanant systems to be two stage regulation (rv's are considered permanant).
Only portable appliances(grills, fish cookers etc) can still use single stage. So if you replace it will be a twin stage.
I agree with the others recycle your cylinder valve slowly to reset the internal excess flow, if that dosent work you may need to replace the pigtail/pol fitting.
EDIT I can not tell from the picture is this a two cylinder set up with an auto change over ? If so then the auto change over head acts as the first stage.
Jim & Debbie England
Do you have Gas? 2015 F350 6.2L CCLB DRW 4.30 axle. 2012 Yellowstone Ridgeline 34RLT Fifth Wheel