86' 6.9L - No frills, manual transmission. The line goes from the pump, to the (vacuum distributor block thing) to the booster. There is one other line off the vacuum block that goes to something near, or possibly is, the horn. My horn doesn't work, and is very far down my list of things to fix.

On my way to the grocery store, a few miles into the trip, and BAM. Hard brake pedal. I'm basically giviner' all the way to get the thing to stop rolling. I can rev the engine and get some pedal back, but never normal pedal. I limp home.

Get it parked and leave it running. Brake pedal is all my leg at this point.
I observe:

1. The vacuum pump is running.
2. When I pull the hose from the brake booster, I have some vacuum.
   1. It kind of pulses - I have no idea if this is normal, or if the amount of thumb suction is appropriate. Was not strong but was not weak. I have felt more on a gas car.
3. I put the hose back on, turn off the engine. 2 hrs later, I pull the hose and it still had vacuum.

Brake booster replaced in October. Master cylinder last month. No problems until now.

It was a torrential downpour, but I can't see how that would affect the system.

Help?

 

Put a vacuum gauge on it..

There $20 from most places

If you have 22+ hg and still no power brakes suspect the booster

 

If your check valve on your booster is plastic check to make sure it isn't cracked. I just changed out mine and that was the problem. Hope you have as easy of a fix..

 

Iam going through the same thing, 86, 6.9. no frills manual transmission etc...
I guess this started about 6 months ago. Suddenly no boost to the brakes.
Checked vacuum and it led to the Booster. replaced it and seemed ok, then
back to the same problem.
Checking vac and it led to the Vac Pump. replaced it and sort of ok.
Brakes would work fine, then suddenly like pushing on a brick.

Unplugging the heater vac lines and checking the vac there, it would drop off,
so plugged off that system....And still having the sudden loss of vacuum.

Put it in a shop, they checked things over, turned the Rotors and drums as they
were a little out of round etc... but they couldn`t find a problem, even taking it for
a test drive.
I leave the shop, get to the corner and no vacuum and pushing on a brick.

It isn`t constant, just every once in a while.
I did pull the check valve on the Booster and it holds a vac sucking on it.

This vacuum sucks.

Charlie

 

With the vacuum gauge on it, can you cycle the brakes several times without loosing vacuum?

If you can't I've had both a weak pump or a leaky booster cause this.

Just went through this same stuff myself...

Edit:Now that I have a few minutes wanted to add my troubles I had.

Four years ago my booster when south where under vacuum it would start applying the brakes by its self. When I took the master off I noticed it was also leaking into the booster just a hair.

$150 later I had a reman booster and a new master. For some reason the brakes just weren't the same as when I had bought it...both were new when I got it and only made it two years...

Well my brake pedal started getting less and less power boost. Over a week I went to a rock hard pedal with absolutely zero power assist.

Hooked my vac gauge to it and as soon as I hit the pedal vacuum would drop to 5hg and take forever to come back up. So I thought weak pump just like last time cool. However the new pump didn't fix it. I bought a new dorman because I needed my truck asap for work. It grenaded before I left the driveway on a Saturday, can't have one until Monday, and it's a reman, said sure whatever.

I took the dorman vacuum pod and put it on my old drive pod. Still no power brakes, vacuum pump still can't keep up...

So I run a vacuum line from my 89 to my 85's booster, to rule out both the pump and the hoses. Still no power assist.

Yank the booster, it's now a special order item, takes a week to come in, master was also leaking when I took it off....had to order that too and it took another day.

Finally my brakes are close to how they were. I can lock up the fronts on pavement. And they're 33x12.50 mud tires.

Go to adjust the rear drums, and the year and a half old wheel cylinder is already leaking...

It just never ends. But I have brakes now and the truck is safe to drive..

I'm pretty disappointed the booster and master only lasted four years, both did have a lifetime warranty, but the down time just wasn't worth it..

 

I`ll connect the vac gauge when I get home from Church and see where Iam at.
Is it normal when the engine is off, and applying the peddle to hear a ssssssssst.
From under the dash? I know that is air rushing to replace the vacuum. Maybe a
seal leaking?

PNP is having a sale of Brake parts this week till thursday, might be some better
parts than the Chicom auto parts we buy today. Actually my booster came from
Mexico as I remember (Remanufactured)

NO Iam not buying parts from PNP.
How ever last week my BIL and I were there pulling parts off 3 different IDI`s,
85, 86 and 89. I did grab 2 of the vac pumps, had great suction.

Charlie

 

Tested vacuum and was at 8hg this morning. Slapped on a new pump - 26.5hg at cold/high idle, and the pedal is back. Have not had opportunity to drive it.

I went with a Dorman pump, hopefully that wont bite me. But AAP is close, lifetime warranty, and I didn't want to wait, as I might need the truck.