Hydrovac Truck Losing Water Pressure: Diagnosis Guide

Nothing's more frustrating than watching your water pressure gauge drop to zero while you're trying to break ground. The vacuum side's working fine, you've got material to move, but without water pressure, you're just standing there. I've been fixing hydrovac water systems for 25 years, and I'll tell you right now — more than half the "pump failures" I get called out for are something simpler.

Let me walk you through how to actually diagnose water pressure problems so you can either fix it yourself or at least know what you're dealing with before you call for help.

Start Simple: Is There Water in the Tank?

I'm not trying to insult your intelligence, but I've driven to job sites where the water tank was bone dry and nobody noticed. These tanks are big, and when you're focused on the job, you might not realize you've used more than you thought.

Check your sight glass or gauge. If you're at or near empty, that's your problem. Fill it up and move on. But if the tank's full and you've still got no pressure, keep reading.

The Number One Cause: Clogged Strainer

Before you panic about pump failure, check your strainer. Every hydrovac has a strainer or filter before the pump — it keeps debris out of the pump internals. When it clogs, water can't flow and pressure drops.

How to Check:

• Find the strainer (usually in the line between tank and pump)
• Shut down the system completely
• Remove the strainer housing or basket
• Look for debris — dirt, rust, algae, whatever

I had a call from a contractor in New Jersey last summer who was convinced his pump was shot. Drove out there to find his strainer was completely packed with algae from using pond water to refill. Cleaned the strainer, back in business. Took 15 minutes.

Prevention Tip: Clean your strainer weekly at minimum. More often if you're using non-municipal water sources. Some guys I know check it every morning — takes 30 seconds and prevents this exact problem.

Check Your Water Pump Prime

Water pumps need to be primed — they can't suck air and create pressure. If your pump loses prime, you'll have the pump running but no pressure building.

Signs of Lost Prime:

• Pump sounds different (higher pitched, cavitating)
• Pressure gauge shows zero or very low pressure
• Pump runs but nothing comes out the nozzle

How to Re-Prime:

• Make sure the suction line from tank to pump is full of water
• Check for air leaks in the suction line (cracked hoses, loose fittings)
• Most pumps have a bleed valve or prime port — use it
• Some systems have electric prime pumps — verify they're working

Lost prime usually means you've got an air leak somewhere in the suction side. Find it and fix it. Common spots: hose connections, cracked pickup tube, bad tank fitting gasket.

Winter Problems: Frozen Lines and Components

If it's cold and your water pressure quit, ice is the likely culprit. Water expands when it freezes, and it doesn't care if it's inside your expensive pump when it does.

What Freezes:

• Water lines (especially exposed sections)
• Pump housing
• Spray nozzle
• Hose reel
• Any water left in the boom

Prevention: I wrote a whole guide on winter hydrovac preparation that covers this in detail. Short version: keep your water system heated, drain it if you're parking overnight in freezing temps, and use the truck's heating system when operating in cold weather.

If It's Already Frozen: Don't try to run the pump against a frozen line — you'll blow seals or crack housings. Thaw it gently with warm water or a heat gun on LOW. Never use a torch. I've seen guys crack a pump housing with thermal shock from hitting frozen cast iron with direct flame.

When It's Actually the Pump

Sometimes the pump really is the problem. Here's how to know:

Worn Pump Internals: High-pressure water pumps run at 2000+ PSI. The pistons, plungers, seals, and valves wear over time. When they wear enough, you can't build full pressure.

• Pressure gradually dropped over weeks/months (not sudden failure)
• Pump runs but max pressure is lower than it used to be
• Water leaking from pump housing or weep holes
• Unusual pump noise (knocking, grinding)

Pump Shaft Seal Failure: This is common. Water leaking from behind the pump pulley means the main shaft seal is gone. You'll usually see a puddle under the pump.

Cracked Pump Housing: Freeze damage, excessive pressure, or impact. You'll see water spraying from the crack. This usually means pump replacement — housings aren't typically rebuildable.

Pressure Regulation Problems

Your system has a pressure regulator or unloader valve that maintains consistent pressure and protects the pump. If this fails, you get pressure problems even with a good pump.

Symptoms of Regulator Issues:

• Pressure spikes higher than normal then drops
• Pressure fluctuates wildly during operation
• System works fine at first, then pressure fades
• Pump cycles on and off rapidly

The Fix: Regulators can sometimes be adjusted, cleaned, or rebuilt. But diagnosis requires a pressure gauge and understanding of your specific system. This is where professional service makes sense — a wrong adjustment can damage the pump or create a safety hazard.

Hose Reel and Nozzle Checks

Before you blame the pump, verify the rest of the system:

Hose Reel Issues:

• Swivel joint leaking (pressure loss at the reel)
• Hose kinked or collapsed internally
• Ball valve at reel not fully open

Nozzle Problems:

• Nozzle plugged with debris
• Wrong size nozzle (affects pressure reading)
• Damaged nozzle diffusing spray instead of focusing it

I've seen operators fight "low pressure" for hours when the nozzle was just partially plugged. Takes 10 seconds to check.

Electrical and Control Problems

Modern water systems often have electrical controls:

Things to Check:

• Pump motor power — is the pump actually running at full speed?
• Pressure switch — does it signal the pump to run?
• VFD (Variable Frequency Drive) — if equipped, is it commanding proper speed?
• Low water cutoff switch — this prevents dry running but can fail in the "off" position

On trucks with electric water pumps (not belt-driven), check your battery and charging system. A weak battery or failing alternator affects everything electric, including pump performance.

Diagnosing with a Pressure Gauge

If you want to get scientific about it, a pressure gauge is your best friend:

At the Pump: Install a gauge at the pump outlet. If pressure is good here, your problem is downstream (hose, reel, nozzle). If pressure is low here, the problem is the pump or something upstream (strainer, suction line, prime).

Normal Pressure Range: Most hydrovac water systems run 1500-3000 PSI depending on setup. Know what YOUR system should produce. Compare current reading to spec.

Pressure Drop Under Load: Some drop when you pull the trigger is normal. Huge drop (like 50%+) suggests pump wear or undersized supply line.

The Quick Diagnosis Checklist

When your water pressure dies, check these in order:

1. Water in tank? (Check gauge/sight glass)
2. Strainer clean? (Pull it and look)
3. Pump primed? (Listen for cavitation, check for air leaks)
4. Frozen? (If cold weather — thaw before proceeding)
5. Pump running at speed? (Belt slipping? Motor issues?)
6. Leaks anywhere? (Visual inspection of pump, lines, connections)
7. Hose and nozzle clear? (Quick check at the end of the system)

If you go through all of these and still can't find the problem, you're looking at internal pump issues or control system problems that need professional diagnosis.

What Repairs Cost

Just so you know what you might be facing:

• Strainer cleaning: Free (DIY)
• Re-priming pump: Free (DIY)
• New strainer/filter: $20-50
• Pump seal kit: $100-300 (plus labor)
• Pump rebuild: $500-1500
• New water pump: $1500-4000+
• Rush emergency service to diagnose what turned out to be a dirty strainer: Embarrassing but it happens

Check the simple stuff first. Seriously.

When to Call for Help

Call me or another hydrovac repair specialist when:

• You've checked the basics and still have no pressure
• Pump is making grinding, knocking, or screaming sounds
• You see significant water spraying from the pump housing
• You're not comfortable working on high-pressure systems
• The same problem keeps coming back

I cover Pennsylvania, New York, New Jersey, and the DC/Maryland area with mobile repair service. Water pump problems are something I can usually diagnose quickly and often repair on-site.

Call me at 272-296-9637 if you're stuck. But try that strainer first — you might save yourself a service call.

For related issues, check out my guides on PTO problems (since the PTO often drives the water pump) and the complete hydrovac troubleshooting guide.