*Central Pneumatic pressure in air line will not stay constant

by Travis
(Madisonville tennessee )

Central Pneumatic 99780 air compressor

Central Pneumatic 99780 air compressor

Hi, I have a central pneumatic air compressor 4hp 17 gallon Model 99780.

The tank will build pressure with no problem but when I start to use my air tools the pressure drops only after a few seconds of running.

I can hear the hose refill with air and it runs fin again for a few seconds.

It started this all of a sudden.

I've adjusted the regulator and have gotten it to return pressure back in the hose faster but it still will not hold pressure constantly.

Thanks for your help!

Comments for *Central Pneumatic pressure in air line will not stay constant

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Nov 06, 2018
Central Pneumatic 99780 99781 regulator fix
by: chris

Pressure in line doesn't stay constant because the little rubber stopper that gets pressed in when turning the yellow knob is worn out. can't find an exact replacement for the rubber stopper.

But I found that the Bostitch Regulator Manifold AB-9415883 Pressure Reducer on Amazon is the exact same mechanism.

Just remove the knob from the housing, then the underlying nut with the rubber stopper beneath on both the original regulator and the replacement. I transferred the whole knob/large nut/push plate and the underlying regulator nut with its new rubber stopper over the the original regulator.

You'll need to use the original longer spring that the rubber stopper is on. voila. I'd post a pic if I could.

(To add photos to this existing post, start a new thread in this same forum, use this same post title and write that the photos are for an existing post with the same title. You can then upload up to four more photos. They will be moved here. Photos can be up to 1.5 MB and 800 x 600 px in size. Moderator)

Dec 31, 2015
3 gal pancake
by: Jeff

My regulator will not hold pressure. When I use a tool after a couple of shots with the brad nailer the regulator gauge drops 15 to 20 pounds.
Any answers?


Apr 19, 2015
On similar Model 99781 Air Regulator was the culprit.
by: Piddler

Model 99781 Air Regulator (built into the manifold)Impossible to find a rebuild kit for it. Harbor Freight customer service tried but could not ID my 6 year old unit. Solved by replacing existing manifold with Couplings and Tees with inexpensive regulator 'in-line' between tank gauge tee and the 'regulated flow' gauge tee. I relocated the pressure relief safety valve to the 1st tee which connects the incoming tank line and tank pressure gauge mounted on end. Had to cut a hole in cover housing. tee is end where Mounting was a little odd but it works fine.

Oct 17, 2014
Picture is up
by: Bill

Travis, I did get your new post, and have deleted them, and added the image here.

Please note that I've put a white circle around where I would tee off the line to see if I got good air pressure to my air tool, to help isolate the regulator to see if air passage through it was the problem.

As the compressor pumps air it flows down this white circled line into the tank. The gauge on the left displays the tank pressure.

With the compressor off, and the tank fully drained, you can put a tee into where the nut on the flex line attaches to the manifold. Add a coupler to the tee, and you now have an air supply to wherever you want it to go that is not affected by the regulator.

If you take the line completely off, and take it, along with a coupler that is the same as the one on your compressor now, you can acquire all the parts necessary to add the tee and coupler from a good plumbing shop. They will likely be brass and / or copper, and that's good.

Make sure you don't take the coupler that's installed on your compressor now, for if you do, you will have opened a path from the tank, through the regulator, to atmosphere, and all tank air will escape, however slowly it might, due to our perceived obstruction in the regulator body.


Oct 17, 2014
by: Travis

I apologize, I was referring to the other comment about the tank being filled with condensation. I had drained that out first.

I'll get some pictures up on a new thread of this ASAP.

I appreciate the help and very quick response!

Oct 17, 2014
Tank full - no air
by: Bill

While this... "Yes the tank has been emptied and the tank shows that it's full." puzzles me a bit, I'm expecting you meant that the gauge shows that the tank is full of air, after you drained it and ran it back up to cut out, yes?

OK, based on what you've written, I'm pretty sure that it is the regulator itself is the problem.

Rather than messing with the plumbing, replace the regulator and see what happens. Worst case, you'll have a spare regulator. Best case, your problem is gone.


Oct 16, 2014
Pressure regulator
by: Travis

Yes the tank has been emptied and the tank shows that it's full. But when I run the grinder for example it loses power. But once I release the trigger you can hear the line refill

Oct 16, 2014
Compressor plumbing
by: Bill

I take it that you, for sure drained the tank?

As for the pressure switch, I cannot see where it is on your compressor as I cannot find an image of it.

Regardless, if the tank is empty when you disassemble the manifold, and you install your coupler upstream from the pressure switch, that will not affect the pressure switch "seeing" the pressure in the tank.

If you start a new thread, and add 4 images of your compressor, one from each side with the shroud off, and refer to your original post name, then I can add the images and we'll all have a better understanding of how to tee off to get air before the regulator so you can see if it is the regulator that is causing the problem.

Oct 16, 2014
Pressure regulator
by: Travis

Ok, once I take the cover off both the pressure regulator and tank pressure gauges are made into one bridge. Would I disconnect the entire bridge? If so will the pump still shut off at a certain pressure?

I hope this makes sense lol. That you!

Oct 16, 2014
by: Carls421

just to add to Bills suggestion, do you drain your tank of condensate? (Water) If the tank is full of water then the capacity of the tank is reduced considerably so you empty the tank of air quicker than the compressor can fill it again.

Oct 15, 2014
Air regulators fail
by: Bill

Among other things on your air compressor, air regulators fail.

If you are handy, tee off the discharge line upstream from the air regulator and feed your air tool with unregulated compressed air for a bit.

When you are using the air pulled before the regulator, are you experiencing the same issue with pressure drop.

If not, you need a new regulator I would expect.


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