B5900 Air Compressor Pump Issues and Repair Steps

by Terry
(East Texas)

This 2 stage compressor pump comes on several models of compressors, such as Porter Cable, Sanborn, Black Max, Coleman, Devilbis and of course Kobalt.

The initial symptom that there was a problem with my compressor was a loud air popping sound coming from compressor while running. Checking around, and listening and feeling, it led me to the safety valve ( pressure relief valve ) on the intercooler tube. Since the compressor appeared to be pumping air just fine, and the popping noise from the relief valve only started about 125 psi, I incorrectly thought it was a bad relief valve. So I ordered a new one of same relief pressure ( 65 psi ). Well, when I installed the new one when it came in, you got it?.. It started popping off about the same pressure, so I knew then that something more serious was wrong. I was beside my self, because I had bought this compressor ( Kobalt 80 gallon 2 stage, 5 HP ) from Lowe?s brand new just 3 ½ years earlier. I can safely estimate that the compressor only has about 35 to 40 hours or less run time. And the warranty ran out at 3 years, so??

The compressor appeared to fail the very first hard workout I gave it, really. Previous workouts were short duration pump ups, and not so frequent pump up requirements. On this occasion, it was hot in the shop, probably 85 to 90 degrees, and I was using a sandblaster that pretty much made the compressor run continuosly for an hour or more while I blasted.

In want to make a point here. The compressor came new, already set up to cut in at 145 psi, and cut off at 177 psi. The whole time I had the compressor, I had been thinking that I ought to lower that pressure, because, for one, I didn?t need that much pressure, and second, I felt that above 125 psi, the compressor and electric motor labored much harder to achieve that 175 psi pressure. But you know hind sight is more precise. After the relief valve replacement , I lowered the air pressure control switch settings for the cut in to be 95, and the cut out to be 125, which is just about where the popping sound came on. I knew this was a temporary fix, so I could continue to use the compressor for a couple of months more.


the meantime, I was on the fence about fixing this compressor pump, and felt that it might be throwing good money after bad, because it had already failed after only a few hours usage, and there would be no guarantee that it would not fail right away if it was repaired. And I have noticed that even Kobalt has discontinued using this compressor pump on their 80 gallon compressors. That says a lot.

And during this time I had thought long and hard about what the problem might be, and had pretty much came to the conclusion that the only way that intercooler tube could be overcharging was for an intake valve on the high pressure piston side to fail, or a head gasket between the two pistons to fail, allowing high pressure air to escape back into the intercooler tube.

Around this time, I found a decent, or at the time believed it to be a decent buy on a used 2 stage Champion R15B compressor. I knew the design and quality of these compressors were top of the line, and well proven. A new Champion compressor pump alone sells for $1200 to $1500 bucks. And the better pumps use a better designed valve system, so I bought it. There were issues, but that is another story.


Anyway, after several months of procrastinating, I have decided to go ahead and repair the Kobalt B5900 pump, and thought I would share a little of my findings and techniques to do a top rebuild.

Dissassembly revealed that the reed valves, though coated with a small amount of oil residue buildup were in good shape, with no wear or distortion. I carefully removed the gasket material from both valve plates, and soaked them in gasoline for an hour or so, then blew dry and set aside.


The pistons and cylinder walls look great, so they were left alone.

The head appeared fine and it was checked for damage, and set aside for reuse.

The culprit was a blown head gasket between the intake side and exhaust side of the high pressure piston ports, and the top valve plate. Note: This is a real weak point in this design, because the only pressure that can be exerted by the head onto this gasket is only what the thin valve plate can take before it bends or warps.

Really poor design in my opinion. Life expectancy of this compressor can?t be much. In my case, I believe the sandblasting demand and hour long or more continous running on a hot day made for a pretty hot compressor, and caused the valve plate metal to get hot enough to bend and distort slightly allowing the weak point between the two piston ports to weaken and allow high pressure air to blast through the head gasket to the lower pressure port. Unfortunately, even after the compressor cools, this is permanent damage, and once blown will always give way under pressure.

Before installing a new gasket set, and reassembling, I believe it is imperative to really examine the head, and valve plate mating surfaces to make sure it can accept a new gasket, without blowing right out on usage. I am enclosing pics demonstrating using a known straight edge across the weak point of the head and valve plate, with a feeler gauge under the straight edge. If you can insert even a .002 feeler gauge between the straight edge and head or valve plate you should discard the valve plate, and buy a new valve plate assembly, and if the head has warpage, though more unlikely, well, it has to be resurfaced flat someway or buy a new head.

In these photos, even though the feeler gauge is under the straight edge, its for demonstration purposes only. On mine, there were no clearance, indicating a flat surface. In other words, my head and valve plate had no warpage. Holding both up to the light with the straight edge across revealed no light coming under, as a second source of checking.


Note: These parts are expensive. The valve plate assembly sells for $110 to $200. bucks, and the head and cylinder gaskets will set you back another $40 to $50 bucks. I?m sure a new head would be out of sight.

All total, there are 3 gaskets sandwiched between the top of the cylinder , valve plates and head. Because of this, I have elected to torque the head bolts in two stages, working the 2 center bolts first, then the four outer bolts in a criss cross pattern, first at 10 pounds torque and then 20 pounds final torque, based on SAE GRADE 5 bolt tightening torque recommendations of 31 pounds. I have no pump manufacturers torque specs, so have to go by bolt size and hardness, which is a 3/8, 16 tpi hardened head bolt. I reduced the amount of torque by 33% because I?m dealing with aluminum head and cylinder, and aluminum expands a lot when hot, and care has to be given on tightening torques. ( I learned this from rebuilding many, many VW and Porsche engines in my younger days ) If I were dealing with a cast iron or steel head and cylinder I would probably go for the full torque recommendation.

By the way, I could not find bore and stroke information anywhere on this pump, so since I have it where I can get that information , I will share it with you guys. Here it is:

Bore - Large Piston - 4.125

Bore - Small Piston - 2.162

Stroke - 2.190

Displacement Low Pressure Piston - 29.267 CU IN
Displacement High Pressure Piston - 8.039 CU IN