The Castair CTD5812 is a 5/8" compression tube fitting inlet x 1/2" NPT male outlet in-tank check valve. It is the part that keeps compressed air in your tank when the pump unloads and prevents backflow when the system is at rest. If your compressor is bleeding down through the head or you need to replace a worn check valve on a Cast Iron pump, this is the OEM-sourced replacement rated for the pressures real shop equipment runs at.
What This Part Does
The in-tank check valve is installed in the tank port where the pump discharge tube connects. It allows air to flow from the pump into the tank during compression, then seals shut when the pump stops or unloads — preventing reverse airflow back through the pump valves. A failed check valve shows up as the pump struggling to restart under load, excessive bleed-back from the unloader line, or the compressor failing to build pressure efficiently.
Rated Specifications
- Rated Pressure: 500 PSIG maximum
- Rated Temperature: 400 degrees F maximum
- Inlet Connection: 5/8" compression tube fitting
- Outlet Connection: 1/2" NPT male (tank side)
- Side Port: 1/8" for unloader tube connection
- Style: In-tank, compression fitting inlet
- Part Number: CTD5812
When You Need This Part
The CTD5812 fits compressors where the pump discharge tube is 5/8" OD copper or steel tubing and the tank boss is tapped 1/2" NPT. This is a common configuration on medium-duty single-stage and two-stage shop compressors. Before ordering, verify your existing check valve's inlet tube OD and the tank port thread size — Castair makes this valve in a full range of inlet/outlet combinations so the right match matters.
Signs the Check Valve Needs Replacement
- Bleed-back through unloader: Air exhausts from the unloader port during or after shutdown for an extended time, indicating the seat is not sealing
- Pump loads immediately on restart: Tank pressure pushes back through a failed valve; the motor restarts against full pressure and trips thermal overload or blows fuses
- Slow pressure buildup: A leaking check valve allows partial backflow, reducing net CFM delivered to the tank
- Audible hissing at tank inlet: Air bypassing the valve seat during pump operation
The Cast Air In-Tank Check Valve Lineup
Castair offers this valve in multiple inlet and outlet combinations to match different pump and tank configurations. Common sizes in the CTD series (compression fitting inlet, NPT outlet):
- CTD3838 — 3/8" compression x 3/8" NPT
- CTD3812 — 3/8" compression x 1/2" NPT
- CTD1212 — 1/2" compression x 1/2" NPT
- CTD5812 — 5/8" compression x 1/2" NPT (this part)
- CTD1234 — 1/2" compression x 3/4" NPT
- CTD3434 — 3/4" compression x 3/4" NPT
- CTD3410 — 3/4" compression x 1" NPT
- CTD1238 — 1/2" compression x 3/8" NPT
NPT female inlet versions (CTB series) are also available if your pump uses a female threaded connection rather than a compression tube fitting.
Installation Notes
- Thread sealant: Use pipe thread sealant on the NPT threads at the tank boss — do not use tape on the compression ferrule side
- Torque carefully: Compression fittings seal with ferrule deformation; overtightening cracks the fitting body
- Unloader port orientation: The 1/8" side port faces the unloader tube — verify clearance before fully seating the fitting in the tank boss
- Pressure test: After installation, run the compressor to operating pressure and confirm no leak at the fitting joints or through the valve seat at rest
Technical Specifications
- Manufacturer: Castair (Cast Air, Inc.) — Spicer, Minnesota
- Part Number: CTD5812
- Type: In-tank check valve, compression inlet / NPT outlet
- Inlet: 5/8" compression tube fitting
- Outlet: 1/2" NPT male
- Unloader Port: 1/8" side port
- Max Working Pressure: 500 PSIG
- Max Temperature: 400 degrees F
- Application: Piston-type oil-lubricated air compressors
Why Buy From Air Compressor Factory Direct?
We stock Castair replacement parts alongside their compressors because sourcing OEM components from a third party turns a 20-minute repair into a week-long parts hunt. We know which check valve goes with which pump configuration. If you are not sure whether the CTD5812 is the right fit for your machine, call us — we will verify based on your compressor model before you order. Free shipping on qualifying orders, free technical support, and the full manufacturer warranty on every part we sell.
Call 866-217-0063 for quick answers and help!
