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@Rubiclone I had this same issue, ended up being a blockage in one of the lines. Rig up compressed air on the line that connects to the vac pump and see how it is coming out by the booster. Also make sure you are using vacuum rated rubber lines, you might have one that is collapsing under vacuum. Start with changing them all to new lines to rule that out.
Great idea, I did replace the main feed line to the booster with a new rubber hose. It seems to be vacuum rated as when its holding vacuum I can still squeeze the line. I have not replaced the small line going to the idle up solenoid so was planning on removing that completely and installing a rubber cap on the fitting on the pump just to rule that out.

Did you happen to run a vacuum gauge on your rig when you had that issue? Did it seem to build boost slowly like mine was showing?
 
@Rubiclone Yes mine was building vac slowly when i was having the issue, it shouldn't do that, thats a sign that there is some restriction going on. If everything is fine it should jump up to 25-30inHG pretty quick.
 
After taking a diagnostics break I have made no further progress. My hose is not collapsing, I don't have a leak in the fitting or the hoses because after about a week of the car sitting in the garage I pulled the small throttle up hose off the solenoid and could hear the vacuum pop. I then questioned the check valve in the pump so removed that core and it made no change. Currently running a new hose right from the pump to the booster bypassing the extra volume can.

***If someone could please post a video of a normal pump flow rate, when the booster is empty. I will send a $20Cad Amazon gift card to the first person to do so via email******
 
1hz has the same thing but tbh I've never needed to investigate it. The vacuum is only for the brake booster in mine (no clutch booster). Does it make any really noticable difference if the vacuum side is not working (or not working well)?
 
This is a process of elimination problem. You've already verified you have vacuum at the pump. To be thorough, verify you have vacuum at each of the fittings coming off the pump.

Then disconnect the circuits and plug them off one at a time. I would start by testing the circuit to the clutch booster (if you have one). I've seen a couple boosters fail, usually going unnoticed because it is totally unneeded.

You can use your lungs to blow air through the lines (just use a clean piece of vacuum hose).

Once you identify the circuit that is leaking / restricted you can replace the rubber line, blow our the restricted hard line with compressed air, or plug off/replace the leaking vacuum-driven device (like a clutch booster or vacuum switch).

Vacuum will build very quickly in the absence of leaks/restrictions.
 
This is a process of elimination problem. You've already verified you have vacuum at the pump. To be thorough, verify you have vacuum at each of the fittings coming off the pump.

Then disconnect the circuits and plug them off one at a time. I would start by testing the circuit to the clutch booster (if you have one). I've seen a couple boosters fail, usually going unnoticed because it is totally unneeded.

You can use your lungs to blow air through the lines (just use a clean piece of vacuum hose).

Once you identify the circuit that is leaking / restricted you can replace the rubber line, blow our the restricted hard line with compressed air, or plug off/replace the leaking vacuum-driven device (like a clutch booster or vacuum switch).

Vacuum will build very quickly in the absence of leaks/restrictions.
Yeah I did all that, I blew through towards the pump, felt some restrictions with the check valve flutter so I removed the innards from the check valve and tried again. No change. I did learn a valuable lesson though and that without that check valve the vacuum from the booster pulls oil into the pump and fills it up ;). I then just blew back into the pump through the main line to push the oil out of the pump. I also removed the small line from the idle up and plugged it to eliminate. After sitting for a week there was still vacuum in the line so I have no leakes upstream of the pump check valve.

It's also an auto so I don't have any vacuum for the clutch. Just the idle up and brake booster
 
Went for a test drive and all is the same. I have one good brake session but if I pump the pedal three times and then step on it I can't lock up the tires. If I'm parked at idle I can pump the brakes 4 times and the low brake light comes on and the pedal is hard.
 
So I pulled the pump apart again and everything looked fine inside. It had quite a bit of oil from my idea of removing the core of the check valve so cleaned it all out and put it back together. Again no change.

Does anyone in here have an hdj81 that could tell me how many times you can press the pedal at idle before the low vac light turns on?
 
Not sure this will help since @Loober and others have already said vacuum should build up very quickly, but here's a quick video showing how fast my 1990 HDJ81 builds vacuum. Vacuum line, in this case attached to output of the pump.

vacuum.mov


Note also, I haven't started this truck in over a week and when I pulled the line from the vacuum pump, it hissed a little, so was holding vacuum. The pedal felt fine for a few pumps (before removing the vacuum line) then was very firm with the line detached. No sign of a low vacuum light...which I didn't even know existed.

Good luck!

edit: Direct link to video here in case it doesn't load above: http://www.dustbird.com/ih8mud/80/vacuum.mov
 

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