Draco, First.........
Welcome to the dirt pile
A few pictures would help, for general discussion about your problem I will start by agreeing with Willyr. Check all your joints for wear. Once you have tightened the blades joints up then it is time to re-enforce the blades structure.
I have what we call around here a Greenville Steel Car blade on my 420 and it has some wear issues that I need to take care of.
Here is the original thread on it.
http://www.jdcrawlers.com/messageboard/ ... c&start=30
You can see in the picture of the brace or toggle link the that the pin is worn badly, not only the pin is worn but the holes in the push frame and the blade itself are worn also making for a loose blade.
All the loose fitting joints add up into slop that can allow the blade to move when the weight of the blade is taken off the hydraulic cylinder.
Check all the pins and pivot points for wear, check any bolt holes for wear also. While you are checking these, watch for broken and cracked welds on the push frame.
Once you have the pivot points repaired move on to repairing the push frame. If you repair any damage on the push frame before repairing the pivot points you can inadvertently pull the pivot points out of alignment.
A lot of 420C's were produced with the single lift cylinders over the years, and they were functional, I got lucky on mine as it has the dual lift cylinders.
Read this link on posting photos.
http://www.jdcrawlers.com/messageboard/ ... php?t=4262
Dan.