54 inch plow problem

I have a X485 with a snow blow on it and when I am plowing with the plow angled and I am pushing heavy snow the blade does not hold the angle ( slips back) the local John
Deere dealer said I should install a shut off valve on the line going to the rear ram. ( I did not get the hydraulic cylinder lockout valve when I got the front quick hitch and lift) He said that would make the angling more of an even movement but was not certain about the angling slippage. Any thoughts? Should I go buy this valve $96 and install it, am I damaging anything when the plow blade slips back?