the pressure falls immediately from around 100psi to 30

by Bob Carden
(Naples, Florida)

My compressor acts like it is not building pressure in the tank. It runs fine and builds pressure but it seems like it is only pressurizing the line.

When I use the air the pressure falls immediately from around 100psi to 30. The motor kicks on and if I stop using the air, will build pressure back up to 100psi within a few seconds.

If I was receiving air from the tank it should take longer for the pressure to fall that far and it definitely should take longer to pressurize the tank. It will pump up tires and run a nail gun while the motor runs but it will not run a ratchet or grinder. It is a 2.5 hp 10 gallon Pro Source. Do you have any suggestions as to what I should look for?
Hey Bob.

Typically, the only things in the line between the tank and the air line is a regulator and the discharge coupler into which you connect the air line.

Your compressor symptoms suggest to me that your air regulator is either set to too low a pressure, or that it is failing, that it allows the downstream air to build in the line slowly, but when you use air, it doesn't allow the flow through at a sufficient level to keep the downstream air pressure at the correct setting.

That's what I think, based on what you wrote.



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Sep 26, 2013
by: Carls421

Do you drain your tank/receiver of water regularly? It is a possibility that it may be full of water hence why it may seem like the tank is emptying very quick?
Just a thought

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Pressure vs CFM at regulator

by George Mihalis
(Kinnelon, NJ)

Compressed air regulator and gauge (Photo:

Compressed air regulator and gauge (Photo:

Does increasing the pressure at your regulator increase the CFM?
That's a really good question, George.

In order for a regulator to lower the downstream air pressure, say from the tank at 100 PSI to 60 PSI which is the pressure you want to run your air tool at.

When your air tool is stopped, the line from the compressor to the tool will be full of compressed air at the pressure set on the regulator. In this case, 60 PSI.

As soon as you start the air tool, the pressure in the line to the tool drops. The regulator senses this and opens quickly, allowing more air flow (that's CFM) into the line, and keeps opening, allowing more flow, until the line pressure reaches the 60 PSI set point. At this point it fluctuates by 3-5 PSI or so depending on the quality of the regulator, as long as you are using your air tool, and as long as you have more than the 60 PSI in the tank.

So yes, when the regulator is turned up, and the air tool is using 80 PSI air instead of the 60 PSI air you had it set at before, the regulator is open wider to allow more flow.



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Step by Step repair of faulty regulator (only short burst of air). Has a defective tank pressure rubber seat

by Ed

Step-by-Step Instructions to Repair Air Compressor with a faulty regulator that does not allow adequate air pressure to come out of air hose.

When tank has plenty of air but you only get a short burst of air pressure (only momentarily) out of air hose and compressor’s air hose gauge goes to zero.

1. Tools Required – Phillips screw driver, a very big straight bladed screw driver, crescent wrench, Dremel, some type of o-ring grease such as Teflon lube, or silicon grease.

pic. 1

2. Be sure tank is not empty and has some pressure in tank. Turn Regulator Control Knob counter-clockwise all the way and release all pressure from air hose connector. The air pressure in the tank forces the Control Knob’s plastic piston to move back into the Control Knob/Regulator’s housing thus preventing damage to the plastic piston when disassembling.

3. Make sure compressor is unplugged.

4. Now release all air pressure from tank using the water drain valve on bottom of tank.

5. Remove (6) screws holding top half of plastic compressor cover on: (2) from handle and (1) at each corner of plastic housing cover. The (2) screws in the handle are Machine Screws 10-32 x 1” Phillips and most likely will strip when reassembled – replace with 10-32 x 1 ¼” Phillips.

pic. 2

6. Lay top half of plastic cover over to side – no need to disconnect any wires.

pic. 3

7. Remove Regulator by using Crescent Wrench and turning hex nut counter-clockwise. Will be tight but be gentle so as not to damage o-ring seal. There will be just enough room to get wrench on at an angle with bottom plastic housing still in place when the Control Knob is completely screwed out.

pic. 4

8. Regulator Control Knob removed.

pic. 5

9. Turn red Control Knob clockwise and the plastic piston with post in the middle (in above picture 5) will be pushed out.

10. Remove plastic piston, compression spring, and metal disk from Control Knob housing and clean all parts with a paper towel or rag; don’t use anything in housing that will scratch inner wall of cylinder, but clean out any corrosion. Do not remove the (2) o-rings from the plastic piston unless you plan to replace them.

11. Turn red Control Knob counter-clockwise again to screw knob out for reassembling.

pic. 6

12. Apply a thin film of lubricant (would not recommend grease as it will gum up with time) - Teflon or Silicon lubricant or o-ring grease to the cylinder walls of the control knob’s housing, to the metal disk, and to the o-rings.

13. Reassemble the above parts. Be sure the metal disk’s cupped side is facing to the bottom / inside of the Control Knob. It is easier to get these parts reassembled if you hold the parts as pictured below (on the crescent wrench) in your hand and place the Control Knob on top of them pressing them into the Control Knob housing carefully, not to damage the o-rings. Press in until completely seated like in picture 5. Sit reassembled Control Knob aside until final assembly.

pic. 7

14. With a large flat screwdriver unscrew brass tank pressure seal assembly. Do not remove o-ring unless you plan on replacing it.

pic. 8

15. Clean inside of manifold with a paper towel or rag.

pic. 9

16. The part causing the problem is the rubber Tank Pressure Seat. Carefully remove the spring with it’s rubber seat, clean both parts. What has happened, due to the type of material, from pressure and moisture the rubber has swelled to the extent that an adequate amount of air cannot pass around the sides of the rubber seat. No need to remove the rubber seat from the compression spring.

pic. 10

17. Using a Dremel with a 1/8” round carbide bit carefully grind a notch into the (4) sides (the center of the sides only, not the corners) of the rubber seat. BE VERY CAREFUL NOT TO GRIND TO DEEP. Upon close inspection of the sealing surface of the rubber seat you’ll notice a circular groove that makes contact with the sealing / mating surface inside of the brass threaded hex bushing, be sure to not grind any where close to this sealing groove in the rubber or your tank will no longer hold pressure when stored. Do Not Grind on the corners at all; the corners keep the rubber seat in place and aligned in the brass hex bushing.

pic. 11

18. Notice depth of the notches is just enough to create a gap for air to come around the rubber when it is reassembled in the brass threaded hex bushing.

pic. 12

19. Apply a very thin film of Teflon or Silicon lube or o-ring grease to the rubber and reassembly into brass threaded hex bushing.

pic. 13

20. Apply a light coating of lubricant to the o-ring and screw brass assembly into manifold. Just tighten snugly – no need to overly tighten o-ring seals -, just good and snug (to tight will cut o-ring).

21. Apply a coating of lubricant to the Regulator Control Knob assembly’s o-ring and also to it’s mating surface of the manifold. This o-ring will be fairly easy to cut if not careful. Reassemble Regulator Control Knob assembly into manifold, again tighten snugly, but don’t over do.

22. Reassembly top half of plastic cover to compressor.

This fix will now allow an adequate amount of air to pass around the rubber seat that had swelled over time.

If someone can locate a part number and a source for a new rubber seat please post it here.
Outstanding post, Ed. I'll turn this one into it's own page shortly.

We all thank you.


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Relieved vs non-relieved regulators - wasted air?

by Marc
(New Hampshire)

I'm looking to replace the regulator on my 11-gal compressor. I mainly use it for nail guns and filling tires. I understand the difference between relieving and non-relieving regulators, but couldn't find info on how they operate.

Suppose I use a relieving regulator and set it at 90 psi. With the tank pressure at 120 - 150 psi, wouldn't the regulator constantly bleed off and empty the tank? Occasionally I fill the compressor, turn it off, and drag it somewhere that's not close to electricity for some small job that's OK with just a full tank. With a relieving regulator, wouldn't I just lose too much air to be of much use?
Actually Marc, the relieving regulator relieves downstream pressure, not the pressure from the tank.

Let's say that you had an application that needed 30 PSI, and not one bit more. If the regulator was set for 100 PSI, then the air in the line to your application would be at 100 PSI too, and when you dialed the regulator down to the 30 PSI, if the regulator didn't relieve the air in the downstream line down to that 30 PSI, then your application would get hit with the 100 PSI already in the line until the pressure vented down to the 30 PSI of the setting on the regulator.

So, take your air compressor with you, running or not, with 100 PSI in the tank, and the regulator set for whatever your nail gun needs, you won't waste air with a relieving regulator.

A lot of folks charge up an air pig to take with them for nailing jobs so they don't have haul around a compressor, particularly up and down stairs.



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Aug 05, 2013
by: Marc

That's the info I was looking for.
You are welcome!


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