# Air compressor - what's the extra small line going to the swtich?



## BlueRidgeMark (Nov 28, 2015)

I have an old Sanborn 5 hp air compressor, and it seized up on me.  

It has the switch mounted to a port on the side of the tank, and I"m assuming that it gets its pressure connection from there so it knows when to switch on the motor.

However, it also has a small line running to the top of the current pump, to a port on the head.  My new pump (a Harbor Freight unit) does not have a port for this.

*Does anyone know what this is for?  * I'm going to assume it needs to be connected to tank pressure somewhere.  I'm thinking of connecting it to the top center 1/2" port where the line from the old pump was connected.    I'm going to add a gauge there because it will be easy to see.   Other than that, it will be unused.  I can easily add a tee with an adapter for the small line.

I plan to connect the new pump to a 3/4" port on the side of the tank, as the output from the new pump is 3/4" and I don't want to restrict it.


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## heat seeker (Nov 28, 2015)

It's probably a bleed line. When the compressor stops, it bleeds any pressure still in the cylinder and line so the motor can start up again without a load. New units don't seem to have that feature; at least mine doesn't. That's probably because the cylinders are smaller these days, so less startup load on the motor. They make up for the smaller size by running faster that the old ones.


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## BlueRidgeMark (Nov 28, 2015)

Okay, got it.   With that info, I was able to do some searching and find out what's going on.

The discharge line from the pump to the tank requires a check valve.  The bleed line must connect between the pump and the check valve, to bleed off pressure exactly as you described.

This allows the pump to start with zero pressure, to reduce starting load on the motor. 

So, I need to find a check valve for my 3/4" discharge line and figure out the fittings to connect my bleed line between the check valve and the pump.

Thanks!  Sometimes a little clue is all that's needed.


EDIT:

Found the valve with a built-in port.   Here's the description:


One new 3/4" Female NPT x 3/4" Female NPT in-line air compressor check valve. This check valve features a 1/8" Female NPT port in the side to go to a unloader valve if needed. This is a universal inline check valve with female threads on both ends. This valve prevents back flow or tank pressure from getting back to the air compressor pump once the compressor is shut off. The 1/8" unloader port on the side of the valve is used to relieve pressure from the pump once it has shut off. If your compressor does not use an unloader, this port can simply be plugged.
For more information click on technical specs.
​


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## heat seeker (Nov 28, 2015)

Yeah, it's just supposed to bleed the cylinder and line. There should be a check valve to keep the air in the tank.


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