You can run a pressure test with a special MB tool and a pressure gauge to determine which pump isn't working to specs, however, this test was only designed to determine premature pump failure when the pumps are relatively new. I would follow the advice biven here by others and replace both pumps if they have been in service for a long time. That being said, you need to determine if the pumps are really at fault. Testing these pumps is not an exact science even though the book says it is. If the pumps are noisy they should be replaced. Also you can hook an amp probe around the wires and read the current draw and wave form of each pump. This is not a foolproof test as I found out. I had a 300E that died on the road and was towed in. Ran great in the shop and on test drives. I discovered no current flow through one pump. I tapped it with a hammer and it came alive with a jagged wave form. I ordered new pumps. When they came, I checked the currnt pattern again just for *****s and giggles. The pattern and current draw was perfect on the failing pump. The conclusion is that if the pump fails any tests you've discovered the problem, but if it passes the tests, you still don't know. Also there are pressure and volume tests. You may want to seek out a professional before you spend money you don't need to. Good luck.

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