Brake booster, QJet secondaries.

[VW_Factor]

Please forgive me. I registered and searched around and didn't quite find what I was looking for.

What I find to be a strange issue, figured I would ask before I start getting into something I don't understand.

My father has had this 1978 GMC Jimmy 3/4 ton since I was maybe 8 years old. He's since passed away, the truck has seen plenty of use and fun. Sat for some years, and this past weekend, pretty much sight unseen, drove it 900 some odd miles to bring it home. (Towing no less)

Anyway. SB 400, with a QJet. None of the emissions equipment is installed anymore.

When I got it fired up and such, the brake booster didn't seem to be working. Brake operated fine, of course very stiff and a lot of effort on the pedal. Got it rolling, towing my car back and it was immediately apparent that the secondaries on the carb are not opening either. (I don't know a whole lot about these voodoo QJet thingys, I'm familiar with Holley's and Webers, etc)

So, lots of driving and never one did the booster come up nor did the secondaries open.

This morning, get up unhitch, sort a bit (got in late) fire it up. Brake booster is operating perfectly, drove it around just a bit. Give it a bit more loud pedal and the tires light up (wet pavement). The secondaries are definitely opening now.

What gives? Can a brake booster be intermittent? How is this related to the secondaries (or is this coincidence?)

Thanks for your time.

 

[diesel4me]

Might be a long shot,but the power brake booster gets its vacuum supply from a fitting on the back of the carb or the intake,if it was clogged up with carbon or something (mud dauber wasps nest? !) it will not allow any assist to the brakes--maybe something came unclogged after you used it awhile..... and perhaps the secondary barrels were just stuck from sitting or the air valve above the throttle butterflys got stuck from its long nap..wouldn't hurt to douche the carb with some gumout or similar product and ensure all the linkages and butterflys open and close properly...some Q-jets had a diaphram that holds the secondaries from opening right away too,that could be faulty..

 

[VW_Factor]

Here soon when I get a bit more time between work and my other project. I'll remove the carb and check out the line between the carb and booster. Clean up the carb a bit as well. The line has got a smallish looking canister thingy. Not sure if that is a filter or maybe a check valve of some sort. (Line to the booster.

So I take it the brake booster won't be an intermittent thing? Its either working or its broken?

If the booster is broken would it cause a vacuum leak?

 

[n4xplay]

If the brake booster was torn or leaking in any way you would hear a hissssss either at stand still or when you tried to apply the breaks
If its hissing when you step on the breaks then the diafram is bad and needs to be replaced if its hising with your foot off the break pedal the seat may be defective old/ misformed you can give this one a bit to see if it comes back but a easy way to check is to pull the vacuum line going to the booster and see if the sound stops identifying the booster as faulty

 

[VW_Factor]

I'll take a listen for that next time I'll have it running.

Thanks for the tip.

 

[diesel4me]

The booster ,if leaking,will usually affect the idle quality and speed when you apply the brakes,and as stated,it can sometimes leak ALL the time,and you'll hear it pretty easily...the "filter" loking thing in the vacuum hose is a charcoal filter ,GM put them there to help prevent oil vapors from getting to the diaphram in the booster and rotting the rubber ,leading to early failure...but many cars and trucks dont have one ,and it doesn't seem to be a big deal...