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Gearcase Pressure/Vacuum Tester
FREE 2-DAY SHIPPING
ALL DOMESTIC ORDERS
Stop water intrusion before it destroys your lower unit. This professional-grade combination tester allows you to pull and hold both vacuum and pressure with a single tool.
- ✓ 2-in-1 Design: Flip the valve to switch between pressure and vacuum testing.
- ✓ Universal Fit: Includes custom brass adapters for Yamaha, Suzuki, Tohatsu, Mercury, Johnson, Evinrude and Volvo Penta.
- ✓ Quick Disconnects: Brass swivel fittings make installation fast and prevent hose twisting.
- ✓ Superior to OEM and aftermarket: 90890-06756, CDI 551-34PV, S-34, V-34.
Built to last by Marine Tech Tools in Charleston, SC. 100% Money-Back Guarantee.
Catch Water Intrusion Before It Destroys Your Gears
Most boaters just drain their gear lube, check for milky water, and refill it. By the time you see water in your oil, the damage to your bearings has already started. This combination Pressure/Vacuum tester allows you to test the integrity of your lower unit seals before symptoms present, saving you thousands of dollars in gearcase rebuilds.
Technical Specifications
| Feature | Details |
|---|---|
| Dual Function | Dual-Action (Flip switch for Pressure or Vacuum) |
| PSI Gauge | High-visibility, highly accurate dial for slow leak detection |
| Connection Type | Brass swivel quick-disconnect |
| Universal Adapters | Mercury, Mariner, Johnson, Evinrude, Force, Volvo |
| Comparable OEM and aftermarket part numbers | 90890-06756, CDI 551-34PV, S-34, V-34 |
The Double-Seal Design
Why do you need a tool that does both pressure and vacuum?
The seals on your drive shaft and prop shaft are actually comprised of two distinct oil seals mounted back-to-back:
- The Inner Seal (Pressure Test): The lip faces inward. Its job is to keep the gear oil INSIDE the lower unit. You test this by pumping positive pressure into the gearcase and watching the gauge to see if it holds.
- The Outer Seal (Vacuum Test): The lip faces outward. Its job is to keep lake/ocean water OUTSIDE the lower unit. You test this by pulling a vacuum (negative pressure) and ensuring it holds.
Testing only pressure is doing half the job. This 2-in-1 tool allows you to test both with the flip of a valve.
How to Test Your Gearcase Seals
Step 1: Drain the Oil
Remove your upper and lower gearcase drain screws and drain the gear lube completely.
Step 2: Connect the Tester
Thread the upper vent screw back in. Thread the appropriate quick connect adapter from the set into the bottom drain hole.
Step 3: The Pressure Test
Flip the valve on the hand pump to Pressure. Connect and pump the gauge up to the manufacturer's spec (usually around 10 to 15 PSI). It should hold pressure for at least 5 minutes. To diagnose individual leaking seals rotate drive shaft, prop shaft and shift shaft by hand. Additionally you can spray soapy water around the prop shaft, shift shaft, and water pump base to to identify leaky seals.
Step 4: The Vacuum Test
Release the pressure. Flip the valve to Vacuum. Pump the gauge to pull a negative vacuum (usually 3 to 5 inches of mercury). Rotate the prop shaft and drive shaft and shift shaft by hand while under vacuum to find leaks. Gear case should hold vacuum for 5 minutes. If you find a pressure or vacuum leak, replace both inner and outer seals on that shaft.
Who is this tool for?
This is a high-quality hand pump set designed for DIY Boaters, Mobile Marine Technicians, and Independent Repair Shops. It offers professional diagnostic capabilities at a fraction of the cost of pneumatic dealer equipment. While high-volume dealership shops testing dozens of lowers a day may prefer a continuous-air pneumatic tester, this hand-pump provides the exact same accurate diagnostics in a portable, self-contained unit.
Customer Reviews
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This is a decent brake ...
May 8, 2025
This is a decent brake bleeder at best. It does not move enough volume to be an effective Gear Case Pressure Tester. We have some more elaborate brake bleeders as well as pressure/vacuum tools. I see this product being returned due to a lack of performance in the advertised task. That said we are a large boat service center and have always been satisfied with Marine Tech Tools. We use a wide variety of their products and couldn't be more satisfied.
