Attention: TDR Forum Junkies 
Sorry for the long post but this subject is twofold. I am also posting it in other categories. Maybe everybody else had seen or heard of this but it was new to me.
The last time that I had my oil changed, the first thing that was done was the guy above removed the oil fill cap from the valve cover and placed their vacuum hose over the opening. He then hollered down to the guy in the pit to remove the drain plug. After which the guy in the pit hollered up to remove the vacuum hose. The oil then was drained.
I immediately questioned the service writer as to what the vacuum was used for. He said that they have discovered that by drawing a vacuum on the crankcase the guy in the pit doesn’t get HOT OIL all over his hands and arms when pulling drain plugs. He also said that they could even pull the drain plug completely out and replace the copper gasket or plug without losing any oil! Neat huh?
Then the light went on! Could I use my wet/dry shop vacuum to draw a vacuum on the fresh water system on my RV to drain it instead of blowing it out with (possibly oily) compressed air? How I got from drain oil to fresh water I will never know!
It works!
I first drained the hot water heater and then replaced the drain plug. The end of my shop vacuum hose will fit over the ends of the low point drains and the vacuum will hold the hose up against the bottom of the RV. I then opened the cold and hot water low point drain valves, this will draw a vacuum on the fresh water system. I then went into the RV and sequentially opened each hot/cold water valve, leaving them open until I could no longer hear any water bubbling in the pipes. I even ran the water pump for a few seconds to assist in draining it. I assume that if your drains are located such that your vacuum hose won’t fit over both of them that doing each drain separately would work just as good.
Try it, I think that it is a lot better
The last time that I had my oil changed, the first thing that was done was the guy above removed the oil fill cap from the valve cover and placed their vacuum hose over the opening. He then hollered down to the guy in the pit to remove the drain plug. After which the guy in the pit hollered up to remove the vacuum hose. The oil then was drained.
I immediately questioned the service writer as to what the vacuum was used for. He said that they have discovered that by drawing a vacuum on the crankcase the guy in the pit doesn’t get HOT OIL all over his hands and arms when pulling drain plugs. He also said that they could even pull the drain plug completely out and replace the copper gasket or plug without losing any oil! Neat huh?
Then the light went on! Could I use my wet/dry shop vacuum to draw a vacuum on the fresh water system on my RV to drain it instead of blowing it out with (possibly oily) compressed air? How I got from drain oil to fresh water I will never know!
It works!
I first drained the hot water heater and then replaced the drain plug. The end of my shop vacuum hose will fit over the ends of the low point drains and the vacuum will hold the hose up against the bottom of the RV. I then opened the cold and hot water low point drain valves, this will draw a vacuum on the fresh water system. I then went into the RV and sequentially opened each hot/cold water valve, leaving them open until I could no longer hear any water bubbling in the pipes. I even ran the water pump for a few seconds to assist in draining it. I assume that if your drains are located such that your vacuum hose won’t fit over both of them that doing each drain separately would work just as good.
Try it, I think that it is a lot better
