Like Dell says, you are getting water in your system. Do you keep the tractor parked outside where rain and snow can get introduced into system? If so and you don't have a barn or shed to park it in, buy a good tarp, I opt for the good canvas ones, not the blue nylon ones, and keep the tractor covered when not in use.
Now, you need to to do a better job of draining system. Then do a visual check of system to see if you can spot a cracked pump base or bad piston. It does sound like your pump may have cracked due to water being in it and it freezes. You fire up tractor to plow snow and though you let it warm up, the frozen pump isn't thawed out and you go to use it and BAM! it busts as the housing is aluminum. If pump isn't cracked and cylinder doesn't leak, it may just need a new safety relief valve which you can change out yourself. There are methods of do-it-yourself procedures to help pinpoint root cause. Email me for more info...