Bad booster or bad vacuum pump?
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Bad booster or bad vacuum pump?
Intermittent brake assist on a '91 IDI, trying to isolate the problem. Shortcut question: do these boosters hiss under normal operation, and if not, can that be caused by anything other than a bad booster?
Steps I've taken so far:
1. Connected vacuum pump line straight to booster line, bypassing the 6-way junction for the cruise control and whatever else. No change, so I rule out problems elsewhere in the vacuum system.
2. Disabled vacuum pump by removing its belt. Manual brakes are expectedly heavy, but work properly. I rule out hydraulic issues.
3. Connected vacuum pump to (and only to) a vacuum gauge. It quickly gets to 19" vacuum and holds ~15" for a few minutes after shutdown. Suggests pump is ok but that's with no load.
4. Connected vacuum pump and gauge to the junction. At idle it's usually but not always ~10-12", getting up to 18 after cruising for a minute. Push the pedal, there's a hiss, vacuum drops ~8", pedal is very soft and the truck stops quick. Vacuum builds up slowly and unsteadily. If I push the brakes again and get vacuum down to 0-2", they become manual again. Oddly enough, I don't notice a significant change in pedal effort between 2" and 19".
Steps I've taken so far:
1. Connected vacuum pump line straight to booster line, bypassing the 6-way junction for the cruise control and whatever else. No change, so I rule out problems elsewhere in the vacuum system.
2. Disabled vacuum pump by removing its belt. Manual brakes are expectedly heavy, but work properly. I rule out hydraulic issues.
3. Connected vacuum pump to (and only to) a vacuum gauge. It quickly gets to 19" vacuum and holds ~15" for a few minutes after shutdown. Suggests pump is ok but that's with no load.
4. Connected vacuum pump and gauge to the junction. At idle it's usually but not always ~10-12", getting up to 18 after cruising for a minute. Push the pedal, there's a hiss, vacuum drops ~8", pedal is very soft and the truck stops quick. Vacuum builds up slowly and unsteadily. If I push the brakes again and get vacuum down to 0-2", they become manual again. Oddly enough, I don't notice a significant change in pedal effort between 2" and 19".
Posting Guru
Joined: May 2015
Posts: 2,036
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Sounds like you've got a compound problem, a vacuum pump that's getting weak and a booster that's leaking which just makes it worse.
I'd do the booster first as it's cheaper and probably the factory one, if you can find a vacuum pump in a junk yard thad save a lot of dough
I'd do the booster first as it's cheaper and probably the factory one, if you can find a vacuum pump in a junk yard thad save a lot of dough
Cargo Master
Joined: Oct 2012
Posts: 3,072
Likes: 48
From: California
His profile says he's outside the USA. Whenever I needed a vac pump while in Canada, it cost a fortune and took forever for the parts suppliers to deliver it. I think the dealer estimated it to be CA$500. I found a Napa that could get one for in 3 days for around $250. No junkyards had these IDI vac pumps. The best option (if you can wait) is to buy one off of RockAuto and save a lot of money.
Posting Guru
Joined: May 2015
Posts: 2,036
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You may find with a new booster that doesn't leak that it might be fine or not even weak for that matter.
Just if you get one and it still hisses. Take it back and get another. Seems like one in three I get don't work properly.
Just if you get one and it still hisses. Take it back and get another. Seems like one in three I get don't work properly.
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Cargo Master
Joined: Oct 2012
Posts: 3,072
Likes: 48
From: California
If he's had a significant leak, I bet his vac pump has been working overdrive for a long time, and it's probably overdue for a replacement. At the very least, he should just buy one and keep it in the truck just in case he ends up in a situation that needs the pump replaced. With a simple socket set, you can change that part out in a typical parking lot if necessary.
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