Hydrovac Won't Build Vacuum - Here's Why and How to Fix It

Look, if you're reading this, you're probably sitting in a dead hydrovac somewhere, watching that vacuum gauge do absolutely nothing while your boss is calling asking why you're not moving material. I've been fixing these hydrovac trucks for over 10 years, and vacuum problems are what I deal with most. Let me tell you what's probably wrong and what you can actually do about it.

First Thing - Is Your Blower Even Running?

I know it sounds stupid, but you'd be surprised how many guys call me out just to find their PTO isn't engaged properly. That little light on your dash doesn't always mean the blower's actually spinning. Get out, pop the hood, and look at the thing. Is the belt moving? Can you hear it? A Roots blower makes a very distinct whining sound - if you don't hear it, you're not building vacuum because you're not even trying to.

Had a guy last week in Allentown swear his blower was bad. Drove out there to find his PTO cable had stretched and wasn't fully engaging. Five minute adjustment, problem solved. Always check the obvious stuff first.

Your Relief Valve is Probably Stuck Open

Nine times out of ten when someone calls me saying their vacuum won't build, it's the relief valve. Especially in winter. That valve sits right on top of your blower - it's supposed to pop open if you pull too much vacuum, keeps you from imploding your tank. Problem is, moisture gets in there, freezes, and holds it open just enough that all your vacuum goes right out to atmosphere.

Here's what you do: Find the valve (big round thing with a spring, usually has a red cap), and tap on it with a wrench. Not hard enough to break it, just enough to free it up. If you can, spray some WD-40 or whatever you've got in there. Work it by hand a few times. When it's working right, you should feel spring pressure pushing it closed.

I've gotten so many trucks running again just by freeing up a sticky relief valve. It's literally a two-minute fix if you know what you're looking at.

Door Seals - The Silent Killer

Your debris tank door has a massive rubber seal around it. When that seal goes bad, you're sucking air instead of building vacuum. The thing is, it doesn't have to look bad to be bad. I've seen seals that looked perfect but were hard as a rock and wouldn't seal worth a damn.

Here's the quick test: Take a dollar bill (or any piece of paper), close it in the door, and try to pull it out. If it slides out easy, your seal's shot. Should have some resistance. Do this all the way around the door.

If your seals are leaking, you can buy yourself some time with heavy grease - just glop it on the seal before you close the door. It's messy and temporary, but it'll get you through the day. I've also seen guys use foam weather stripping from Home Depot as a temporary fix. Again, not pretty, but when you're trying to finish a job, pretty doesn't matter.

When Your Blower's Actually Bad

Now, if you've checked all that and still got nothing, your blower might actually be worn out. These PD blowers have tight tolerances - we're talking thousandths of an inch between the lobes and the housing. After a few thousand hours, that clearance opens up and you can't build vacuum anymore.

Put your hand on the blower housing after it's been running for a few minutes. If it's too hot to keep your hand there, you've probably got worn lobes. That extra clearance causes blow-by, which creates heat. A healthy blower runs warm but not scorching hot.

Also, check your blower oil. If it looks like chocolate milk, you've got water in there, probably from a bad seal. That'll kill a blower faster than anything. These things need clean oil changed every 200 hours or so. I see guys run them 1000 hours on the same oil then wonder why they need a $10,000 rebuild.

The Expensive Stuff

Sometimes it's not a simple fix. Worn blower lobes need to be rebuilt or replaced - that's a shop job requiring micrometers and specialized knowledge. Cracked blower housings can sometimes be welded but usually need replacement. Damaged drive gears inside the blower mean a complete rebuild.

Had a Vactor last month where the owner kept running it with low oil. Seized the whole thing solid. That turned a $200 oil change into a $12,000 blower replacement. Don't be that guy.

What You Can Actually Do Right Now

If your vacuum won't build and you need to get moving:

Start the truck, engage the PTO, and watch that gauge. Nothing? Get out and check if the blower's actually turning. If it is, find that relief valve and free it up - seriously, this fixes half the problems I see.

Still nothing? Check your door. Open it, look at the seal, close it, and see if it's actually sealing. You can hear air leaks if you listen - sounds like a giant vacuum cleaner with a hole in the hose.

If none of that works, check your emergency shut-offs. Some trucks have a manual emergency valve that dumps all vacuum - make sure it's closed. Sounds obvious but I've driven an hour to find someone accidentally hit the E-stop.

When to Call for Help

Look, I make my living fixing these trucks, but I'm not going to tell you to call me for something you can fix yourself. But if you hear metal-on-metal grinding from the blower, see oil pouring out, or that blower housing is literally glowing red - shut it down and call someone. You're about to turn a repair into a replacement.

Same goes for if you've tried everything here and still got no vacuum. Could be internal blower damage, could be your PTO's slipping internally, could be electrical. At that point you need someone with the right tools and experience.

Keep This From Happening Again

Change your blower oil. Seriously. It's the cheapest insurance you can buy. Check your door seals every morning - takes 30 seconds and saves you from being stuck. And when it's freezing out, cycle that relief valve a few times before you start working. Keeps it from freezing stuck.

I've been doing this long enough to know that most hydrovac problems are preventable. The guys who call me for emergency repairs are usually the ones who haven't touched their blower oil in six months. Don't be those guys.

If you're stuck right now and can't get it figured out, call me at 272-296-9637. I cover PA, NJ, NY, and DC with mobile service. But honestly, try the relief valve first - you might not need me.