The Hydraulic Jack That Wouldn't Hold: What I Got Wrong About Bleeding, Specs, and Enerpac Power Units

In March 2023, I approved a purchase order for an Enerpac hydraulic power unit I was absolutely certain would fix our shop's biggest recurring headache. Three weeks later I was on the phone with our supplier, explaining why we needed to send it back — and being politely told we couldn't. That's roughly $4,400 I still see in my sleep. I'll get to it.

But first, the problem most people actually think they have.

The Problem You Think You Have

"How do you bleed a hydraulic jack" is one of the most searched questions on the internet for a reason. I was one of those searchers — probably four or five times between 2019 and 2022. Every time a jack got spongy, or lost pressure under load, or just refused to lift the way it used to, I did what everyone does: I opened the release valve, pumped it a few times, topped off the fluid, and called it fixed.

And it worked. Sometimes for a week. Sometimes for three months.

Here's the thing I didn't understand for a long time: bleeding a hydraulic jack is a symptom treatment. It's the equivalent of topping off your oil every two weeks instead of figuring out where the oil is going. The air you're bleeding didn't get in there by accident.

Standard bleed procedure, for the record, is roughly this: fully lower the jack, open the release valve, remove the fill plug, and pump the handle slowly through a full stroke 8–12 times to push trapped air out the reservoir. Refill with the manufacturer's specified fluid to the correct level, replace the plug, close the release, and cycle the ram under light load. That's it. (Should mention: use the fluid Enerpac or whoever made your jack actually specifies — mine wanted ISO VG 15, and I used whatever was on the shelf. That was mistake number one, circa 2020.)

If you're bleeding the same jack every few weeks, you don't have a bleeding problem. You have something else.

The Problem You Actually Have

Air gets in through places you're not looking

The first real culprit is almost always the seals. Hydraulic seals degrade, and when they do, air gets drawn in on the return stroke — invisibly. You bleed the system, it feels fine, and by the time you're under load again, the seal has pulled in more air.

The second culprit is wrong fluid. This kills me because it's so avoidable. I said "top it off with whatever's in the drum." What I meant, in my head, was "compatible hydraulic fluid." What the guy in the shop heard was "whatever's in the drum." We were using the same words but meaning completely different things. Discovered this when I finally read the spec sheet and realized we'd been running a mix of two different viscosity grades for eleven months. Every seal in that cylinder was angry about it.

Pneumatic parts are not hydraulic parts

This one hurts because I watched a colleague do it, and I didn't say anything. He was servicing our air tools and ordered parts he thought would also fit a hydraulic setup — because in his head, "air tools parts" and "hydraulic tool parts" are the same category of thing. They are not.

Pneumatic systems run on compressible gas at, what, 90–150 psi in most shops? Hydraulic systems run on incompressible fluid at thousands of psi. An impact wrench — I'll use the TW1000 as the example, because that's the one he was working on — is a pneumatic tool. It has a motor driven by compressed air, o-rings that see a certain set of pressures, and seals engineered for gas. The moment you put pneumatic-spec components into a hydraulic circuit, you're one pressure spike away from a failure mode that sprays hot oil at 5,000 psi. That's not a metaphor. That's a hospital trip.

I also wasted about $340 on air tool parts that sat in a drawer for two years because I thought they'd cross over. They didn't.

The spec sheet was written for a different shop

Here's the part that surprised me — and probably the one nobody tells you, because it makes the manufacturer sound bad and it doesn't.

When I bought that Enerpac hydraulic power unit, I read the specs carefully. Pressure rating, flow rate, reservoir capacity, motor voltage. All of it checked out for our application. What the spec sheet didn't — couldn't — account for was our duty cycle. We weren't running a single jack for ten minutes. We were running three cylinders, on a fixture, in a cycle, eight hours a day with a two-minute pause between cycles. The unit was rated for pressure but not for that rhythm, and the reservoir got hot enough that we were seeing thermal variance in performance by hour six.

I went back and forth for two weeks on whether to rebuild the old unit or spec a larger one. Rebuild meant $1,200 and a specialist we couldn't get for a month. Replace meant $4,400 and a shipping window. I chose replace, because the project couldn't wait a month. That was probably the right call. What I got wrong wasn't the decision — it was that I never should have been in that position. A single question to the distributor — "what's the continuous duty cycle rating vs. the intermittent one?" — would have told me to buy a different model.

Which brings me back to the $4,400.

What It Actually Costs

The power unit I bought didn't fail. It worked exactly as specified — for intermittent use. Our continuous operation drove it into thermal shutdown by hour six every day. We ran it that way for three weeks because we had to. Total cost of the whole episode: $4,400 for the unit, $890 for expedited shipping on the replacement, roughly $2,100 in the first week's lost production while we figured it out, plus whatever I should price a damaged professional reputation at.

Eight, nine thousand dollars, and a week of my team standing around asking why the fixture wasn't running.

Looking back, I should have called the distributor before the PO, not after. At the time, I thought I knew the spec sheet well enough. But knowing the numbers and knowing how the numbers interact with your operation are two different things.

Oh, and the near-miss. During the week we were troubleshooting, one of our guys reached under a partially-lifted load with the release valve cracked — because the jack wouldn't hold and he was trying to see if the cylinder was the problem. That's a six-inch reach from a very bad day. Nothing happened. But I think about it.

I have mixed feelings about brand name pricing. On one hand, Enerpac costs more than the off-brand alternatives we tried in 2021, and I've complained about it. On the other hand, the Enerpac cylinder that replaced the cheap one has been in service three years without a bleed, so the math isn't as obvious as it looks at first.

What I Do Now

Three things. That's it.

1. Continuous vs. intermittent duty cycle — I ask, in writing, before every PO. If the distributor can't tell me, I don't buy.
2. Verified fluid spec — the exact grade the manufacturer states, logged on the jack, with a dated top-off record. No more "whatever's in the drum."
3. Authorized source only — for Enerpac Tool Group gear specifically (they're a publicly traded industrial tools company — you can look up Enerpac Tool Group on Wikipedia if you want the corporate history), I only buy through authorized distributors. Not because of warranty. Because the counterfeit problems in the gray market are real, and I've watched a shop next door learn that the expensive way.

For reference on the standards side: portable jacks in the US fall under ASME B30.1, and the manufacturer's rated capacity and marking requirements are not suggestions. If a jack's overpressure protection is bypassed, that's not a hack. It's a liability.

And one more thing — the small-order thing. When I first started in this role (2017, if we're being honest about how long I've been doing this badly), I used to order single jacks, single cylinders, one hose at a time. The distributor who took my $180 order seriously, walked me through the bleeding procedure over the phone, and didn't act like I was wasting their time — that's the one I now spend twenty grand a year with. Small doesn't mean unimportant. It means they haven't decided yet who they're going to be.

Step one for anyone reading this: before you bleed that jack again, ask why it needed bleeding in the first place. The answer is almost never the jack.