Help me diagnose the issue; Almost no brakes, causes surge

[99'Subourbon]

Hey guys,

I can't seem to track down the culprit on this one.

The problem started Saturday afternoon, after a morning of light off-roading in the mountains near Heber, AZ. When we were leaving to go home, after stopping and eating, I noticed while backing out that when I pressed the brakes, they were stiff all the ways to the floor, and only if I literally STOOD on the brake pedal could I get it to start barely braking. Very slowly, it comes to a soft stop.

Here are some symptoms:

Engine randomly either surges 100~ rpm, or drops a couple 100 rpm, intermittently and randomly, while pressing the brakes down.

While pressing the brakes down, I can hear air, or hissing, coming from behind the dash.

I have checked the vacuum line fro behind the throttle body to the vacuum booster drum, and it get's pressurized, and there is no apparent leaks, while the engine is running or while pressing the brakes. I have checked around the brake fluid reservoir for any leaks or fluid, none. I have check the lines from the pistons up on the front brakes and there is no visible signs of fluids, or blowouts.

Side note, not sure if it matters, some water did get into the intake box when I was going through a large pool of mud/water (although it appeared to be very little, and I doubt it affected anything), and at some point the intake tube popped of the throttle body (S&B Intake), that I only realized once I popped the hood after the brake issues started.

I haven't check the rear drums, or any of the lines going to them - do the drums operate from the brake lines as well? or is it mechanical?

Please help! - Thanks,

 

[michael hurd]

Sounds like you have a brake booster issue. The brakes will be hard when you have no power assist, but they will still stop the vehicle. Surging is a sign of a vacuum leak, and when you are stepping on the brake pedal ( and hear a hiss ) you have additional air that is bypassing the MAF sensor.

 

[99'Subourbon]

Sounds like you have a brake booster issue. The brakes will be hard when you have no power assist, but they will still stop the vehicle. Surging is a sign of a vacuum leak, and when you are stepping on the brake pedal ( and hear a hiss ) you have additional air that is bypassing the MAF sensor.

How can I test the brake booster?

Would a vacuum leak be caused by a failed brake booster? And where would air bypass the MAF?

EDIT: After doing some reading, it's most probable the hissing sound I hear is the brake booster diaphram has ruptured. The diaphram typically acts as a air tight seal between the cabin side and the engine side of the brake booster, and when it's blow, and pressing on the brakes, the air is passing the diaphram and being released into the atmosphere instead of creating pressure to assist with the brakes.

So it is seeming you hit the nail on the head - thanks! Now just got to figure out how to test it. Local vato-zone has them in stock, although I don't know if they are open today.

 

[michael hurd]

Yes, the booster can be a cause of a vacuum leak, when the diaphragm ruptures.

http://www.agcoauto.com/content/news/p2_articleid/129

 

[99'Subourbon]

That was an informative read. I have bad cats that I still haven't replaced (rather removed), which could be contributing to the issue (or helped expedite the failure of the brake booster). I'm going to do some of the tests they mentioned.

Thanks,