Need help with air compressor pump (2-stage)

[Schurkey]

I have what amounts to a Coleman/Sanborn air compressor, using the two-stage SC-23 pump.

The 75-psi safety "pop-off" valve on the intercooler (between the low-pressure exhaust valve and the high-pressure intake valve) releases when the tank is about 2/3 pressurized--maybe 125 psi. If I hold the valve closed with a tyrap, the compressor will squeeze it's rated 175--180 psi into the tank. Clearly, keeping the valve shut isn't safe or recommended.

I have another 75 psi pop-off valve on order...but...I'm coming to suspect that the pop-off valve is OK, there's a problem with the compressor pump.

There are five valves in the cylinder head. Two inlet valves for the low-pressure cylinder, one inlet valve for the high-pressure cylinder, and an exhaust valve for each cylinder.

I'm thinking that perhaps the inlet valve for the high-pressure cylinder leaks, which allows excess high-pressure backwards into the intercooler, which blows the pop-off safety-valve off it's seat.

I have an exploded view of the pump, along with part numbers.

HOW DOES THE HIGH-PRESSURE INTAKE VALVE COME OUT? I removed one of the low-pressure intake valves. There's a threaded plug that holds it in place. Removing the plug required some cleverness, as the wrenching method uses a tool with two pins which fit into two holes in the plug. Once the plug was out, the retainer-cage and then the valve lift out by hand. The valve was covered in gritty carbon--but appeared to be working. I cleaned it and re-installed. I don't know where the gritty carbon comes from. Always has an air filter. Must be burnt compressor oil. Oil level never drops that I noticed.

Over the high-pressure intake valve--the valve that I think is leaking--there's a bolt that holds a heavy washer (Valve cover) to the cylinder head. I can remove the bolt, but the washer won't come off. I don't know what the bolt threads into--apparently it threads into the screw-in plug that secures the cage over the valve. I suppose the valve is also covered in gritty carbon, and won't seal any more. Perhaps it will if cleaned.

Exploded view here:
http://www.mastertoolrepair.com/air-compressor-pump-parts-040-0211-sc23-pump-p-48.html
Pop-off valve #73. High-pressure intake valve #62, retainer cage #63, plug #67. Bolt and "valve cover" shown above the #67 plug.

 

[Schurkey]

Answering my own questions: Yes, I had a leak at the high-pressure cylinder intake valve. The large-diameter copper sealing washer failed. This over-pressurized the intercooler, and caused the safety pop-off valve to...pop off.

The valves are all of similar but not identical design. They're held in place with a big plug that presses on a perforated-steel cage. The valves seal against the head with a copper sealing ring. Three of the valves use a heavy washer as a "valve cover" which is sealed with a paper gasket around the outside, and a small copper sealing washer where the retaining bolt goes through.

Threaded plug and fabricated "Special Tool". Pin C-to-C is 27 mm or very close--slightly more than one inch.

The ******* packet of three paper gaskets, three small copper washers, and five large copper washers is $50. Note broken sealing washer.

Assembly of the high-pressure intake valve.


I removed and cleaned four of the five valves. I couldn't get the high-pressure exhaust valve out of the cylinder head. Operation of the compressor assured me that the valve I couldn't remove...isn't leaking.

I did have another complication: A wayward piece of sealing ring fell into the compressor, and ended up jamming the one-way check-valve leading into the tank. I had an unending leak at the unloader housing (hisssssssssss) as long as there was air in the tank. I had to remove the main (big) copper discharge pipe between the aftercooler and the check-valve at the inlet to the tank, the small-diameter copper pipe leading from the one-way valve to the unloader housing, and then the one-way valve itself. Once out in the open, it was easy to remove the wayward fleck of sealing ring, and all was well again.

 

[redmondjp]

Thanks for updating this thread with great pictures of the tool and parts! Glad you got it working. Classic sign of second-stage inlet valve problem when then interstage relief valve pops off.

That's the one downside of the higher-end compressors with individually-serviceable valves - they often need a special spanner wrench to disassemble.

 

[Schurkey]

I'm about one step away from tossing a street-Tee and a pressure gauge on the intercooler. where the pop-off valve is. Seems like a cheap diagnostic aid for the future. Gauge low = primary cylinder problems; gauge high (and pop-off valve releasing pressure) = secondary inlet valve/seal problems.

 

[redmondjp]

I'm about one step away from tossing a street-Tee and a pressure gauge on the intercooler. where the pop-off valve is. Seems like a cheap diagnostic aid for the future. Gauge low = primary cylinder problems; gauge high (and pop-off valve releasing pressure) = secondary inlet valve/seal problems.

A great idea, much like installing a fuel pressure gauge on a modern diesel engine with the tank-mounted lift pump - it's the only way to tell if your lift pump has failed.

 

[Schurkey]

Photo of cylinder head, with valves labeled.
1 = First Stage
2 = Second Stage

I = Intake
E = Exhaust

For the record, I re-used one of the paper gaskets, all three small copper rings, and four of the large copper sealing rings. I've got enough parts left-over to fix it again.