Just wanted to thank everyone for the good suggestions and insight on this.
I haven't had a chance to bleed the system yet but I also haven't had any
further occurances - though I've used that back heater minimally since then.
I'll check the cap, bleed the system, and tripple check for leaks.

Thanks again

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Thanks,

Jeff
'90 Carat (Grover)
'86 (We call this one Parts)
'85 GL (Preparing to move to a new Home)
'78 Bus (Melissa) Patty's Bus