The Mystery of the Lost Vacuum

Repeated failure of a new pump system has writer stumped. So he pulls into the Truck Corner for help.

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We had a mechanic put a new pump on our vacuum truck. It ran for a few hours and then the check valve inside the pump broke. We took it back and he reported it was most likely a faulty pump part. So he replaced the check valve and we went back to work. It wasn’t long afterward that exactly the same thing happened and the part broke. We started to doubt the mechanic’s ability. What do you think is going on here?

Richard Riley

Lake Havasu City, Ariz.



When these kinds of problems occur, it is because the rpms on the pump are set too high or too low; more often than not, too high. The check valve is a protective device inside the pump to hold the vacuum once the pump is turned off. When the check valve breaks, it is allowed to spin and the vacuum is released rendering the pump useless.




We had the mechanic double-check his rpms on the pump and it was running “hot” as he said. He reduced the rpms and installed a new check valve. All appeared to be well, but upon using the truck again, we had more problems. Whenever we stopped running the PTO, which operated the pump, we could hear a noise and the amount of vacuum was slowly dropping without any valves for suction or elsewhere being open. Normally, our mechanic is pretty good, but this last month he’s been on a roll. What do you think?



If you have solved the check valve problem, there are two remaining places where the vacuum could be leaking. One is the vacuum relief valve. If it is stuck in the open position, you will not retain vacuum for long. So check if your vacuum relief valve is fully operational and doing what it is supposed to do. It should only be opening when the vacuum in your tank has reached a preset level (we usually recommend around 20 to 22 inches of mercury).

If your vacuum relief valve is operating properly, we must look at your mechanic’s work. To replace the check valve, he had to remove at least one of the pump’s end plates. When replacing the end plate, he had to again seal the end plate(s) with the gasket. A gasket that has been damaged or isn’t sealing properly will allow vacuum leaks.

Whenever you open a pump to check interior parts, always check the condition of the end plate gaskets. Over time, they can deteriorate and lose their shape. When this happens, they are not going to provide the proper seal to efficiently operate your pump.

It makes sense to have extra gaskets on hand, as well as a rebuild kit. It is relatively easy to maintain your own pump. The basic rebuild kit consists of a set of vanes, bearings and gaskets. Each kit is specifically designed for the pump, so make sure you order the appropriate kit.



Whenever opening a pump, check the condition of the following:

• The rotor. This is the main shaft holding the vanes. It is held in place by the bearings. If you wait too long to change your bearings, the ends of the shaft can slowly wear down.

• The vanes. Over time, these also will wear down and create less vacuum.

• The bearings. You’ll recognize the sound of your pump changing slightly when the bearings begin to go.

• The gaskets. Check for warping or losing their shape in any other way.

• The interior of the pump. Visually inspect the cleanliness and the condition of the cylinder where the vanes rotate. This area should be free of debris and if it has received the proper oiling every day, should show little wear. Any discoloration is a sign of possible trouble. Any debris within the cylinder indicates that something is happening between the primary and secondary. Perhaps the secondary hasn’t been emptied before operating the pump each day.



You shouldn’t have to get inside your pump often, but these simple checks can save you time, money and frustration. The vacuum pump is the heart of your truck; take care of it.


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