Unable to locate what I think is the pressure relief valve located under the on/off box. looks like a small needle valve but leaks air constantly. any ideas?

An unloader valve when talking compressors, is used with engine driven compressors when it would be impractical to stop the engine when maximum pressure is reached and restarted when it drops significantly and so the system is designed to keep running and when the maximum working pressure is reached the unloader valve sends compressed air to the special compressor cylinder head where the piston of a small pneumatic ram lifts the compressor intake valve from it's seat preventing it pumping until the pressure falls.

This system is also used with electrically powered units when for reasons of light weight and/or economy the electric motor is under-powered. The unloader valve keeps providing air to the compressor head until the pressure falls which is inevitable as there is invariably leakage...

Adequately powered electrically powered compressors do not need the unloader valve system and instead use a pressure switch that if correctly fitted does not leak or at least should not leak. If your unit leaks listening through a short length of hose will help locate the area of the leak and a little soap solution will narrow the search even further (disconnected from the mains power).

Clearly there is an air leak and ignoring the (obvious) possibility of a leaky tank or drain valve or exposed pipework it leaves two possibilities - either the electrical pressure switch diaphragm is failing or the compressor nonreturn valve is failing.

I suggest you switch on the compressor and when air receiver has a partial charge disconnect the power and attempt to locate the leak.
Use a couple of feet of hose as a listening aid for large leaks and a little detergent solution to pinpoint small leaks...

Some pressure switches have a valve that reliefs the compressor head pressure when the motor stops and closes the head pressure relief valve when the motor runs.
There is a check valve that keeps air pressure from coming back to the compressor head and the relief valve.
I think the check valve is leaking tank pressure to the relief valve and compressor head.
The check valve is where the pressure switch mounts on to and it mounts threaded on to the compressor tank.
Try replacing the check valve

check and test the capacitor or power source --to long or not correct extension cord --to small a breaker for load start

Turn on the compressor and look for the air leak. It could be a relief valve or a connecting hose or tube. You must locate the leak then see if it is a part that can be replaced.
Gary

Hot water pressure relief valves serve two functions. The first is to remove high-pressure water from the hot water tank. The second function is to act as a high temperature valve to remove water that exceeds a predetermined point for the valve. Both of these relief settings are preset at the valve factory. Many pressure relief valves will come with a metal tag that identifies these release points. If you're attempting to remove the hot water heater pressure relief valve, pay careful attention to ensure that you replace with the correct valve.

Your hot water heater has a pressure relief valve safety mechanism. When pressure inside the water tank exceeds certain limitations, the valve opens up to allow the pressure to be released. A faulty pressure relief valve will not release excess pressure, which could cause extensive damage to your home. You should test the hot water heater pressure relief valve once every six months to ensure proper operation. If the pressure relief valve fails the test, replace the valve with a new one immediately

Locate the pressure relief valve on your hot water heater. The pressure relief valve has a lever handle on top with a drain pipe attached to it. The pressure relief valve is most often on the side of the water tank toward the top. Newer models of hot water heaters have the pressure relief valve on top of the water tank.

Inspect the area around the pressure relief valve for signs of leaking water. If there is leaking water near the relief valve connection, replace the relief valve. If no leaking is evident, proceed with testing the pressure valve.

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Examine the area below the pressure relief valve drain pipe. Most water heater closets either have a drain in the floor below the pipe or the pipe is inserted into another pipe to direct the water out of the home. If the end of your drain pipe is not in another pipe or there is no drain in the floor, place a bucket under the drain pipe to catch water when you test the pressure relief valve.

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Pull the lever handle on the top of the pressure relief valve up with your fingers. A working lever will lift up without difficulty.You should begin to hear water going through the drain pipe. Slowly release the lever to shut off the pressure relief valve. The pressure relief valve is good. If you did not hear water running through the drain pipe, lower the lever on the pressure relief valve and replace the valve. If the lever is difficult to raise, do not force the lever. Replace the pressure relief valve.

Now with all that said if you have a closed loop water supply system and you have a pressure vessal/expantion tank it may have gone bad and is making the TP valve leak. you can call your water provider to see if you have closed loop system.

Water heater should be set in a drip pan.
Otherwise, put 1 gallon bucket under TP release pipe to catch water.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#drip-pan

When you say relief valve, I assum you are talking about brass valve located on top of tank or near top of tank. This is TP valve and a pipe connects to this valve and runs down to within 6" of bottom of tank. TP valve has a small lever on it.
If you are talking about tank drain valve located at bottom of tank, then add a comment.
Following information is for TP valve.

Dripping TP valve can be caused by 3 different thing:

1) Obstruction inside TP valve. Open TP valve for a moment to flush away particles and slowly lower lever back down. See if water stops dripping, if not, then replace TP valve

2) Defective TP valve: replace TP valve. Turn off electricity to water heater. Let heater cool for several hours so you do not get scalded. Close shut off valve located above tank on cold side. Open tub spout to relieve pressure, open tank drain or open TP valve and empty water below TP valve level. If TP is located on top of tank, not much water has to be drained. Tank is under press when hot, so be careful. Put teflon tape on new valve and install with big pipe wrench by turning clockwise until TP valve is very tight.

3) Overheating tank. TP valve is temperature-pressure valve. If tank water is over 150 degrees, the TP valve starts drizzling hot water to relieve tank pressure.
Lower tank pressure two different ways:
a) turn down thermostat
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
b) install expansion tank on incoming cold water line
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#expansion-tank

Many of the excel units don't come with a pressure relief valve but instead a thermal relief valve, it really depends on the model number of the pressure washer. You should only replace the thermal relief valve if it is leaking.

I want to be sure we are talking about the same. Most likely your compressor has only one safety valve however since it can come with two I want to be sure which one you are referring to. The first will be located on the bottom or back side of the pressure switch. The second if installed is located on the tank check valve. I assume you are referring to the one on the switch correct?

If this is the case then please let me know exactly where this cap was located. You stated it was located under the relief/safety valve but it's not part of the valve correct?
Is it part of the manifold attached to the bottom/backside of the switch?
Although the pic is a generic switch it will help if you can tell me if the location A or B is where you think it came from.
The black line is there because a lot of switches have a flat bottom and some have a small cap to seal off an additional port/ports.
Most switches are not produced by the compressor manufacturer. Because of this many are used on a number of different models as well as a variety of compressor manufacturers. To accommodate as many as they can switches sometimes have ports that are sealed off using a cap or large allen head plug when the compressor does not utilize all the options available on the switch. This is most likely what the cap is for.
Still I want to be sure so again take a look at the pic and if possible give me a general location.
I'll be waiting your reply.












http://www.hitachipowertools.com/product/files/pneumatics/compressors/EC12_E4_BD.pdf

hello, have you checked your pressor relief valve to make sure it has'nt gone bad it's located on the bottom of your tank.