John Deere 458 Baler Help

An issue has recently come up with my 2009 458 standard baler.  The standard has no monitor, just bale shaping indicators on the baler itself.  I recently noticed the shaping indicators would suddenly fall all the way to the bottom rather erratically.  Upon inspection, the tailgate was opening during baling relieving pressure from the indicators.  

 

I have baled around 20 more bales and the tailgate continues to open (sometimes about 1/2 inch, sometimes a couple inches it varies) and occasionally it doesn't open at all.  I've tried to inspect the tractor (2012 JD 5083E) for hydraulic leaks and can't find any visible.  This problem showed up all at once.  Even when the tailgate doesn't open, something funny is still happening.  After baling through the "full" alarm, the alarm often comes back on when tying.  This has never occurred before.  

 

This has me stumped. The manual references "check the pressure relief valves"  "replace if defective".  I have no idea what to check related to the pressure relief valves.  Are these valves serviceable?  Any ideas on what to check on them?  Any help is greatly appreciated.  Thanks in advance.

John Deere 458 Baler Help