Andrew,
I realize that I am resurrecting a very old thread but I stumbled upon these documents that I believe relate to your Oil Pressure Relief Valve.
There is mention of the modification in "Carriers -Minutes of the 4th meeting held at D.T.D. Chobham on 11th July, 1944" located at this link: (images 3348 - 3356) (I have also attached an image)
http://heritage.canadiana.ca/view/oo...5779/1?r=0&s=1
There is a test report on "Cold starting test on pressure relief valve for oil cooler of T 16 carrier, developed by the Ford Motor Co"
Located at this link (images 3432- 3442):
http://heritage.canadiana.ca/view/oo...5779/1?r=0&s=1
I have not tried to analyze the issue to determine if your system is operating as intended.
Colin