So the first (most obvious but sometimes overlooked) thing - check to make sure the tank drains are closed.
Listen for a leak at the pressure switch - it will continue to leak after the compressor shuts off. If this is the case then you need a check valve (Part # 53A on the breakdown)
If it builds pressure but not enough to shut off and there are no leaks I would look at the valves located in the head (Part # 14 on the breakdown) - piston rings are not a normal failure part on this model but could also cause this.
SOURCE: ec12 hitachi air compressor will not build
most likely the the reed valves are stuck or miss aligned. Worse case senerio your piston seal blew causing this not to build pressure. Read manual on how to disassemble head. to remove should be 4 bolts and your copper line, just be careful of the reed valve plates, they are thin steel that sit into the head. Put back the same way you took it apart.
SOURCE: EC-12 runs but only builds to 35 psi and continues to run
so i came to this web site for help. where is the help? are we troubleshooting? come on man! what is this? my compressor runs and runs but does'nt build up pressure. it aint the pressure gauge, dummy. its is a problem with some kind of valve that is supposed to kick on- or off when the pressure gets low enough. problem is which valve and how to fix it! and to the guy whose compressor hums, dont use more than 25 ft of cord, and use the correct size cord and the correct power source.
SOURCE: Hitachi EC 12 Air Compressor does not keep pressure
to repair this problem (a) the pressure switch is not working,(b) check valve or non return valve sticking(stripe down repair and refit)
SOURCE: My 1 hp. 12 gal. air compressor is not building
It could be that the input air filter is badly clogged. I might more likely be that the Non-return valve on the air tank input is not sealing properly and not allowing the tank to build up pressure. This valve is at the end of the main pipe runs between the compressor and tank. It could also mean (less likely) that the poppet valves inside the compressor cylinder head are not sealing properly. Other faults could be worn rings on the compressor pistons or a bad seal between the compressor cylinder head and block. To check the Non return valve allow compressor to build up some pressure and then loosen union nut on Non return valve. There should be some air released at first but should stop if valve is ok. If leaking remove valve and check for dirt and clean. If valve still leaks replacement is the only fix.
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