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Rebuilders vacuum-test valve body bores for one reason: a worn bore leaks, a leaking bore drops line pressure to a clutch, and low pressure burns that clutch. You cannot see bore wear with your eyes. You can measure it with a hand vacuum pump. Sonnax publishes vacuum-test specs for dozens of critical valves, and the tool that lets you use those specs is a $30 hand pump most people already own for brake bleeding.
The idea is simple. Cap a passage, pull vacuum on the valve bore, and watch the gauge. A good bore holds vacuum near the target. A worn bore bleeds down fast because ATF (and air) sneaks past the valve. That number tells you whether to reuse the valve, install an oversized Sonnax valve, or scrap the body. This page covers which pump to buy and how to read it.
What the Vacuum Number Means
The test is done at 200 RPM equivalent by hand, at the specific port Sonnax calls out for each valve. Pull the pump to the target and watch how the gauge behaves. General thresholds most rebuilders use:
| Reading (held) | Bore Condition | Action |
|---|---|---|
| 18 in-Hg or higher | Good, minimal wear | Reuse OE valve |
| 10-17 in-Hg | Moderate wear | Install oversized / Sonnax kit |
| Below 10 in-Hg | Worn, leaking | Ream and sleeve, or replace body |
Always check the Sonnax spec for the exact valve you are testing. Some valves have their own published pass number. The table above is the general shop rule of thumb for when a bore has enough wear to matter.
The Pumps Worth Buying
Mityvac MV8000 Automotive Test Kit — The Shop Standard
Mityvac is the name rebuilders say when they say "vacuum tester." The MV8000 (and the older 8500 selectline) is a pistol-grip hand pump with a clean, readable gauge that holds vacuum without creeping. It comes with the adapters and caps you need to seal off ports. The gauge holds steady long enough to actually read a bore that is bleeding down slowly, which the cheap pumps struggle with. If you are going to do this more than once, buy the Mityvac.
Check Mityvac MV8000
Mityvac MV8500 Selectline — Higher-End, Two Gauges
The Selectline gives you both a vacuum and a pressure gauge and a longer hose kit. If you also do brake and cooling system work, this is the one pump that covers valve-body testing plus everything else. Costs more than the 8000. The extra pressure side is not needed for bore testing but it is nice to own one tool that does it all.
Check Mityvac MV8500
Budget Hand Vacuum Pump Kits — Occasional Use
The generic pistol-grip vacuum kits (often sold as brake bleeder kits) will pull vacuum and can technically do the job for a one-off rebuild. The weaknesses are a gauge that is hard to read at the low end and a pump that does not hold vacuum as steadily, so a slowly-leaking bore is harder to distinguish from a tight one. Fine for a hobbyist doing a single valve body. Get the Mityvac if precision matters.
Browse hand vacuum pump kits
How to Vacuum Test a Bore
- Clean the valve body first. Wash it and blow it dry. Any ATF film in the bore will give you a false-good reading by temporarily sealing wear.
- Find the Sonnax test port. Their vacuum-test guide shows exactly which passage to cap and which to pull on for each valve. Do not guess the port.
- Push the valve to its tested position and seal off the passages that are not being tested with your fingertip or supplied caps.
- Pull the pump and read the held vacuum. Compare to the Sonnax spec or the general thresholds above.
- Test dry, then reconfirm. A bore that fails dry is worn. If it passes dry it will only be tighter once assembled with fluid.
What Goes Wrong
- Reading valve bodies wet. Residual ATF seals wear and hides a bad bore. Always test clean and dry.
- A pump that will not hold vacuum. If the pump itself leaks you cannot tell it from a leaking bore. Cap the pump hose and verify it holds before you test.
- Wrong port. Every valve has a specific test passage. Pull on the wrong one and the number is meaningless.
- Ignoring a marginal bore. A 12 in-Hg bore will still burn a clutch under load. Sleeve it or install the oversized kit; do not "watch it."
Valve Body Worn? Sonnax Kits Ship Same-Day
Once you find the leaking bore, you need the oversized valve or reaming kit to fix it. gearboxinsider.com stocks Sonnax valve-body kits, boost valves, and separator plates. Same-day ship before 2pm Eastern, free shipping, no code needed.
Shop Gearbox Insider →FAQ
Do I really need to vacuum test on a home rebuild?
If you are chasing a specific problem like a burnt clutch or low line pressure, yes. Bore wear is the hidden cause of repeat failures. If you are doing a clean, low-mileage unit and just replacing frictions, it is optional but still smart on the pressure-regulator and TCC valves.
What vacuum reading passes?
Check the Sonnax spec for the specific valve. As a general rule, 18 in-Hg or better is good, 10-17 means install an oversized kit, and below 10 means the bore is worn enough to leak pressure.
Can I use a Mityvac for other jobs?
Yes. The same pump bleeds brakes, tests EGR and vacuum actuators, and checks fuel and cooling components. It is one of the more-used tools on any bench.