Gravedigger
ArboristSite Operative
The 2-stage pumps operate in all positions. The two places air can get into the system are low fluid level in reservior and hose between reservior and pump. The rest of the hydraulic system is under pressure which can leak fluid but not suck air.
A larger oil tank would be a good upgrade along with a filter if you're investing in a new pump. The one gallon tank is honestly too small. I feel 5 gallon tank would be good, although I went with a ten gallon which is a little over kill