by Terry
(East Texas)

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

B5900 pump

B5900 pump

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 continuously 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.

Disassembly 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


by: Anonymous

One of your problems here that causes the pump and motor to overheat, and blowing the head gasket is even though a motor is rated continuous, it still has a duty rating that the pump needs to rest.

I have one compressor with the same pump.

You can figure these pups usually at 65% efficiency and duty rating(yes even though rated cont.) and need to rest the pump…Even rated at 75% efficiency here is how you figure this at 75%…that means you pump the pump for 3/4 an hour…which is 75%…and then rest for the other 25% of the you 45 minutes work time, then at least 15 minutes rest time per every hour.

I sandblast every day and retired this b5900 pump a long time ago for “hard stress”operations..I now use A y design 2 stage pump from Eaton..and even Morpower have good pumps.

Actually the b5900 is a light built pump with reed valves/finger valves, the same thing and they are quite inferior. If you been running that pump like that..that is where the blown gasket comes from, and the head may even be warped for they are not solid cast iron,and are built

Thank You Terry
by: Wayne

Thanks for the reply. Before I ask the follow up questions let me say what I need the compressor to do. The primary function is to run my air sanders. Possibly two at a time at an average of 4CFM @ 90PSI each. If I limit the compressor to 125PSI do I really need a two stage pump? There are many single stage pumps much cheaper. Say like a 5HP 145PSI for less than $200. By the way the best price I find on a new valve plate assembly for the B5900 is $278.


Air Flow Required
by: Terry


I’m a believer in keeping the air pressure down, even on a 2 stage pump. Especially the cheaper ones. Like the B5900. Case in point, I actually have a Champion air compressor with the R15B pump , well actually the whole Champion setup, Champion tank and accessories. Even though that compressor will handle 175 psi easily, I bought a Puma 125 psi air pressure control switch and replaced the original 175 psi switch, because I don’t have any tools that need more than 125 psi, and the electric bill for the extra amps the motor pulls kicking in at 145 psi and shutting off at 175 psi compared to kicking in at 90 psi and shutting off at 125 psi is very apparent. The motor is probably pulling 22 amps at the higher pressures and 12 amps at the lower pressures as stated above.

If you could get a quality 2 cylinder single stage pump that could produce at least 125 to 145 psi that would be great, but pay close attention to air flow ratings at 90 psi. A small 2 cylinder pump could produce 145 psi and have a small air flow and that would not do in your case.

I guess it really comes down to what you can afford. You can get burned by going too cheap. If you run 2 sanders requiring 4 cfm together near continuous, you need a pump that can stay ahead of the game. And not get so hot it fails on you, like apparently the B5900 pump did.

Bottom line for you, there is no way I would spend that much money on that pump. It would fail again the first time it got real hot. Its head design is just too bad.