Properly check vacuum.

[72gmc4x4]

How do you properly check vacuum for 30 min?

If the vacuum drops after closing valves on manifold gauges, does that mean vacuum leak? That's what I thought but someone was telling me it always drops when closing valve on gauges.

[Dougflas]

If you have 29 inches of vacuum while the pump is running, when you close the hand wheels, you should still have 29 inches of vacuum.

[72gmc4x4]

That's exactly what I thought. The gauges will still show 29in of vacuum otherwise you got a big leak..

Thank you.

[wptski]

That's exactly what I thought. The gauges will still show 29in of vacuum otherwise you got a big leak..

Thank you.

That's about all too!

[72gmc4x4]

So....

I pull a vacuum. It gets to 25 inches and then I close the knobs on the gauges and it quickly goes to 10 inches.

I assume leak. I put 125lbs of pressure with nitrogen and it hold for 20 mins...

What am I doing wrong?

[Dougflas]

Check your gauges and hoses. Take the high side hose and place the ends on the high siide looped tothe low side. Pull a vacuum and test. Do this for all of the hoses.

[72gmc4x4]

They won't loop without an adapter so this is what i did.

Low side hose and quick connect adapter connected to gauge but not AC, pull vacuum and opened low side knob. Came down to 28 or so inches quickly. Closed knob and it held.

Moved high side hose to low side and repeated pulling of vacuum. Also held...

I don't know what is going on.

[72gmc4x4]

Maybe not holding vacuum. I came back after about 10 min and the high side hose had lost some vacuum.

I'm going to retest.

[Dougflas]

you do not need adapters. Maybe I did not explain it well enough. Take your low side hose off the manifold. Now your manifold has 2 hoses on it (middle and high side) take the end of the high side off and out it on the low side manifold nipple. Draw a vacuum thru the guages. (Place vacuum pump on middle hose. Find which hose is not holding by exchanging the hoses until one does not hold.

[72gmc4x4]

I think I do understand what you are getting at. I attached a crude drawing of how to test. I just returned the gauges because I think they were faulty.

Please don't laugh.

This particular set of gauges has 2 different types of hoses.

The center has two female ends. The low and high side have one male and one female end.

So I can't connect the low to side to the high side on the gauges without an adapter.