TroubleshootingTech Tips

Proportioner Pressure Imbalance: How to Tell an A-Side Problem From a B-Side Problem

Pressure imbalance is one of the most common calls we get. The good news: the gauges usually tell you which side is the problem. Here’s a practical, step-by-step way to narrow it down.

ArmorThane Tech Team 3 min read
Proportioner Pressure Imbalance: How to Tell an A-Side Problem From a B-Side Problem - ArmorThane Shop

Key takeaways

Summary
  • When pressures split, the side with LOW pressure is usually the one not getting or holding material; the HIGH side is often seeing a restriction downstream.
  • Start with the simple checks: inlet strainers, material temperature and viscosity, transfer pumps and drum levels.
  • A plugged impingement port in the gun’s mix chamber shows up as HIGH pressure on that side.
  • Worn pump seals or check balls cause a side to lose pressure on one stroke direction — a classic sign of a pump rebuild.

Your gauges are the best diagnostic tool on the rig. When A and B pressures split apart while spraying, the pattern tells you a lot about where to look. This guide walks through a practical order of checks — simplest and most common first — so you can get back to spraying on-ratio material faster.

Safety first: Always relieve system pressure and follow your machine’s lock-out procedure before opening any fluid fitting, strainer or pump. Material under pressure can inject into skin.

Step 1: Read the gauges while spraying

Note which side is higher and which is lower, and whether the split is steady or changes with the pump stroke.

  • One side LOW: that side is probably not getting enough material, or can’t hold pressure.
  • One side HIGH: that side is probably pushing against a restriction downstream — often at the gun.
  • Split that changes on each stroke: points toward a pump check valve or seal on the low side.

Step 2: If a side is LOW, check the feed

  1. Drum level and transfer pump. Is the drum nearly empty? Is the transfer pump cycling smoothly and fast enough?
  2. Inlet strainer. A packed inlet screen starves the pump. Clean or replace it — see the A-side Y-strainer assembly and screens.
  3. Material temperature. Cold, thick material feeds poorly. Condition drums to the recommended temperature.
  4. Supply hose. Look for kinks, crushed sections or undersized hose.

Step 3: If a side is HIGH, check for restrictions at the gun

A high side is usually trying to push through something that’s partly blocked:

  • Mix chamber impingement port partially plugged on that side — clean it with the correct size drill bit for your chamber.
  • Gun screen or check valve on that side packed with debris or cured material.
  • Manifold valve on the gun not fully open.

Quick test: Spray into a waste container and watch the gauges. If the imbalance changes when you clean the gun, you’ve found your restriction.

Step 4: Check the pumps for wear

If the feed is good and the gun is clean but one side still can’t hold pressure — especially if the gauge dips when the pump changes direction — the pump’s seals or check balls are likely worn. That’s when a rebuild kit makes sense. PMC and Graco both publish pump rebuild procedures for their machines; we stock kits such as the PMC size 120 A-side kit and size 40 B-side kit.

Step 5: Don’t forget heat and viscosity

Primary heaters and heated hose keep both materials at spray viscosity. If one side isn’t reaching set point — a failed heater element, a bad hose thermocouple or a loose connection — that side can run thicker and drag pressure. Compare actual temperatures to set points on the display.

Quick reference table

Symptom Most likely causes First check
A or B pressure LOW Starved inlet, cold material, transfer pump, worn pump Inlet strainer and drum temperature
A or B pressure HIGH Restriction at gun: mix chamber port, screen, check valve Clean the gun on the high side
Split changes each stroke Worn pump seals or check balls Pump rebuild inspection
Split after warm-up Heater or hose-heat issue on one side Compare temperatures to set points

Still stuck? Call a technician at (417) 831-5090 with your machine model and the pressures you’re seeing — that’s usually enough for us to point you in the right direction.

General troubleshooting guidance only. Always follow your equipment manufacturer’s manual and safety procedures.

Frequently asked questions

Which side is the problem when pressures split?

As a rule of thumb, a side that drops LOW is not getting or holding enough material (starved inlet, worn pump, cold material). A side that climbs HIGH is usually seeing a restriction downstream, such as a plugged mix-chamber port or screen at the gun.

Can cold material cause pressure imbalance?

Yes. Material that is too cold is thicker and harder to feed, which can starve a pump. Keep drums at the manufacturer’s recommended temperature and let heaters reach set point before spraying.

When should I rebuild a proportioner pump?

If a side loses pressure on only one direction of the stroke, or the pressure drops when the pump changes direction, worn seals or check balls are likely. That’s usually the time for a rebuild kit.

Need a hand?

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Our team has built, sold and serviced spray equipment for decades. Call or message us and we'll help you get it right.

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Written by

ArmorThane Tech Team

The technicians and product specialists behind ArmorThane's spray equipment, coatings and applicator training. We write about what we see in the field every day.

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