AC manifold gauges defective?

Just purchased a AC manifold gauge to test low and high side pressures and for adding freon if needed.
For those that have used them, I’d like to see if you have run into this issue.
After hooking up the low and high side couplers to the AC fittings and turning the hose valve couplers clockwise to open (the manifold blue and red knobs are left in the closed position), taking the readings, I then turn the car off, turn the couplers hose valves off by turning them counter clockwise and remove the low side first then the high side connector. The low side comes off easily with barely a whisper of pressure, but when I remove the high side coupler, it spews, forcefully for approx. 2 to 3 seconds, freon from the hose coupler, that was built up in the hose!
Is that normal???
Seems like such a waste of freon…glad I don’t have longer pressure hoses.

Yep, that normal. Just to be sure, you are using the valves that are on the end of the hose where you hook it into your system?

Yes my gauge set has valves at the connectors that hook on to the ac high and low fittings.
So there is no way to avoid such a loss of freon? It creates quite a cloud of freon as it spews out.
I saw a professional using his set and when he removed them…nothing. That’s why I wondered about my set.

I don’t have this problem with my gauge set. I may get a very small burst of pressure that lasts far less than 1 second, probably something similar to what you are getting on the low side.

I have a similar set up on my hoses and when the knobs are turned to back off the valves and the quick connect is released there is a very insignificant amount of discharge.

It might be possible that the high side valve did not close completely for some reason. There is a hose that has volume that has been charged to 170 psi or higher, so if the valve is open, there will be a discharge. If the valve is very close to the quick disconnect, their may not be much of a discharge during the disconnect, but when you open the valve, you will get the discharge described.

In the case of my set, are you suggesting that I didn’t fully close (CCW) the valve (or the valve doesn’t close well) and thus the discharge? If it had been fully closed, then I guess the pressure would have remained in the hose and registering in the high side gauge until I disconnected the hose from the manifold, in which case I would have still gotten the discharge…I assume.
In the case of the pro that I saw using his at the AC shop, his didn’t have valves at the end of the hoses, just couplers. Could his hoses automatically close when released from the fitting thus no large discharge? If so, I would assume that the high side pressure is still in the hose and still registering in the high side gauge until he some how releases it.
I wonder if that is why FordMan59 and Rod Knox see no large discharge on disconnect, because the freon and pressure is still trapped in their AC manifold gauges and hoses.
I guess there is no proper way to release or sucked the freon and pressure back into the AC system before disconnect.
Just trying to make sense of this, in case I have a defective unit.

I’ve never seen a pro set of gauges without a valve at the quick disconnect (QD). The good ones are built right on top of the QD.

I have one older set like this, and on that one open is CW, close is CCW. On the other two, open is CCW and close is CW, but the valves are in line a couple inches away from the QD.

The high pressure hose is going to come off more violently than the low side simply because it is under more pressure. The gauge is going to remain at the last pressure reading until the hose is discharged.

If you got this discharge when you disconnected and the gauge went right to zero, then the valve was open a little. With my latest set, I get a small discharge when I disconnect because of the couple inches of hose between the valve and the QD, then I get the discharge you described when I open the valve to discharge the hose and return the gauge to zero.

Yes, there are QD’s that automatically close when disconnected. If you want to discharge the hoses and zero the gauge, you do it through the charge port.

Thanks Keith,
Mine have the valve on top of the QD, so I guess it doesn’t totally close when I turn it off, thus the large discharge at disconnect. Glad to know it is working properly, although I hate wasting the freon and discharging it into the atmosphere.

If I am just checking to see if the refrigerant is ok, I only hook up the low side with the AC off. The pressure should be around 100 psi, then I start the AC, the low side drops to about 40, then I close the valve and disconnect. Less loss this way.

A manifold gauge set for R134a refrigerant shouldn’t allow any of the refrigerant to escape into the atmosphere. If any of the valves do leak after disconnection there’s a problem with that valve on the gauge set.


There really isn’t much loss on the Low Side, although you still need to release the pressure, it was the High Side that spews like Old Faithful.

Even if my High Side didn’t leak when I disconnected it, how would you get the built up pressure (and freon) in the high side hose and gauge out to “Zero out” the gauge again? There has to be a way to vent the gauges, I would think, in order to get a proper reading the next time you use it. Or am I missing something here?

Unless I’m misunderstanding the question, the next time you hook it up to any system, the retained pressure in the gauge lines will equilibrate to the system pressure. If the line pressure is less than the gauge, it will flow into the system until the pressures equalize. You shouldn’t need to “zero” out the gauge beforehand. If you’re not using the gauges very often, it wouldn’t hurt to vent them (did I say that out loud?). If you’re doing this as a job, there’s no need to vent them between uses and it’s likely to be illegal to do so.

Twin Turbo:
Great answer.
For the casual DIYer I can see v…ing (my secret code word) to prevent any long term issues with the gauges. For the everyday user…makes a lot of sense not too.

Twin Turbo:
By the way, another site forum had a good idea and suggested that normal High side has lots of pressure so wait before removing until gague reading falls.
I realize the high side will not zero out but might fall to a lower psi pressure as some of the freon is sucked back into the system as it equalizes. Kinda what you said about equalizing pressures.

I think the pros aren’t using the manifold gauges anymore, I could be wrong, but I don’t think the EPA allows them to use them for this very reason. All I’ve seen in shops of late is the machine that connects into the system.

It can measure the system, pull it down or refill it and I’m pretty sure it clears the lines without venting to the atmosphere. I don’t have one of these, they run about $2500, a little high for home use.

Pros still use manifold gauge sets. But it sounds like the OP purchased a cheap manifold gauge set. The EPA requires that when the service valves are closed and removed that no refrigerant escape into the atmospher from either the system or the gauge set. That’s the whole purpose of the valves. The refrigerant still in the hoses on the gauge set is removed by connecting the gauge set to a reclaiming machine and removing the refrigerant.


If I took my AC manifold gauges to an AC shop, do you think they reclaim the refrigerant for me? As I said, I don’t like venting into the atmosphere if at all possible. I’m not looking to get it back, just keep it out of the air.

Bring it to an A/C shop and they’ll reclaim the refrigerant for free and give your gauge set back. Takes about ten seconds.