The MPV is stuck closed or barely opens. Air can't leave the separator vessel, so pressure builds up with nowhere to go.

What you'll see

Sump pressure keeps climbing while downstream system pressure is low or zero. The compressor is producing air but it can't get past the MPV. In extreme cases, the MPV body itself can burst from the pressure -- I've seen cases where the separator overheated and hoses connected to the aftercooler were damaged.
Before you assume this is the problem

If downstream pressure is normal, the MPV is opening. Check for separator clogging or inlet valve issues instead.

See all causes of safety valve blows / too high pressure →

How to diagnose

  1. Compare sump pressure with system pressure

    With the compressor running loaded, check both gauges. High sump pressure + very low or zero system pressure = MPV not opening.
    Result: Large gap (sump high, system zero) = MPV stuck closed.

  2. Inspect the MPV

    Stop and depressurize the compressor. Remove the MPV from the separator vessel. Check if the piston moves freely and the spring isn't broken or seized. Look for corrosion, sludge, or debris preventing the valve from opening.
    Result: Piston seized or spring broken = rebuild or replace.

How to fix it

  1. Re-kit or replace the MPV

    Clean, replace all O-rings and seals, verify the piston moves freely. If the valve body is damaged or corroded beyond repair, replace the complete valve. The MPV sits on top of the separator vessel -- easy to access on almost every machine.

Common mistakes

A stuck-closed MPV is less common than a leaking one, but much more dangerous. Don't confuse it with a clogged separator -- both cause high sump pressure, but with a stuck MPV the system pressure is near zero, while with a clogged separator the system pressure is just lower than normal.

Parts & tools

MPV overhaul kit or complete replacement valve.

Review safety precautions before starting →

Safety

A stuck MPV can lead to dangerous overpressure in the separator vessel. If sump pressure is climbing abnormally, stop the compressor immediately.

This issue can also cause

  • Low Pressure / Can't Reach Setpoint Compressor runs and loads, but pressure stays below setpoint: often caused by leaks, restrictions, or capacity...
  • Oil Carry-Over Excessive oil carryover contaminating downstream air: usually a separator, oil level, or temperature problem.