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Derek Spudich

Why does my Craftsman Air Compressor stop building pressure at 30psi?

I own a Craftsman Upright 12 gal. tank, 1 HP Air Compressor (oil lubricated). Model number 921166400. I have had this for about 3 years now with no problems. I use it very sporadically, and it can go months at a time without any use at all. Yesterday, while the compressor was running, the sound of the compressor changed slightly, and it sounded like it started to run a little faster. After several minutes, I noticed that it was not shutting off and the tank would not build pressure past 30psi. I also noticed a slight burning smell, but unsure if that was related to the problem or just from the heat of the motor running longer than normal.

I have drained the tank completely and started the compressor again. It takes a 5-6 minutes (which is a long time to start with) to build to 30psi. It will keep running but will not build past that mark.

When the compressor is off, it will hold 30psi and no leaking is noticed anywhere.

So, whats wrong with my compressor??? Gaskets? check valve?

Thanks for your responses.

Air Compressors , Craftsman , Tools
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4 Answers from these members:
afritz afritz
It can be frustrating when something so frustrating like a air compressor stops building pressure. It defeats the whole purpose of even having one. While researching your question here on Manage My Life I noticed a previous similar question with a posted response from an expert. I have attached the link below for you to view while you wait on your expert response. I hope the link that I have provided you is useful.
by afritz afritz Earned 13,820 community points in Craftsman
November 12th, 2010
Answered in 27 minutes
0 votes
Manage My Life
Your air compressor probably has something broken inside the cylinder. Disconnect the power to the air compressor and remove the plastic cover around the motor and compressor.

Before you take the compressor apart, listen and feel for air leaks. Check to see if air is coming back out of the intake filter. If air is coming back out of the intake filter, something is wrong inside the cylinder.

You can see a parts diagram at []. The parts diagram will help you locate the parts that build air pressure. These are the parts you need to inspect.

Turn the motor shaft by hand and be sure the piston goes up and down.

If the piston is ok remove the 4 screws holding the head on top of the cylinder. Remove the head.

Directly under the head is the pressure plate. The pressure plate has reeds on it that direct the air into the discharge line and keep the air from going back through the intake side of the compressor. Check the reeds for warping or breaking. If the reeds are damaged, you must replace the pressure plate. Make note of how the pressure plate sets and the o-rings that seal around it.

Remove the pressure plate and inspect the cylinder walls for scratches or wear, if the cylinder is damaged you need to replace the cylinder, piston, and rings. Make note of the piston and the rings on it. If a ring is worn or broken the air goes back through the intake filter and no pressure is built. If you have to replace the rings, replace the piston and sleeve also.

To replace the rings and piston, you have to remove the nuts that hold the piston to the eccentric.
by Manage My Life
November 12th, 2010
Answered in 4 hours
0 votes
Derek Spudich
Thank you jimmy K! I have it apart now, and will update you if I find anything wrong. Seems like a fairly simple device, doesn't seem that it should be that hard to figure out the problem. Seems the pressure plate and reeds are all intact, and the cylinder wall looks great. I noticed some pieces of blue material on top of the piston though.

Air was coming out of the intake, and one of the reeds seemed like it was misaligned when I took off the head, but I could have inadvertently moved it when taking off the head as well. Do the reeds just sit in there, or are they supposed to be attached in some way?
by Derek Spudich Earned 22 community points in Craftsman
November 15th, 2010
0 votes
Derek Spudich
It was the head gasket! If someone has this problem, check this first. Follow the directions above. Very easy to get to for anyone that can turn a socket wrench. This has to be a very common problem, very thin area between the intake and exhaust ports where the gasket can (and in my case did) fail. Thank you very much!
by Derek Spudich Earned 22 community points in Craftsman
November 23rd, 2010
0 votes
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