warthogcrewchief

Just recently purchased a '97 C1500 Suburban. The brakes would be extremely sensitive, so I changed out the front brake pads in the event they were contaminated.

While doing so, I bled all four wheel cylinders starting from the farthest from the master cylinder. Brakes were then soft.

So, I bled the master cylinder, then all four wheel cylinders again.

The rear pads are low, but not worn beyond limits, and will change them later.

With the engine off, the brakes are firm. With the engine on, it's extremely soft.


I was thinking it might be a booster because I saw the front plate flexing while my wife was pressing on the brakes. I did some ops checks: 1) held the brakes down while starting the engine - brake pedal dropped to the floor slowly. 2) ran engine, turned off engine, then pressed brakes - brakes progressively firmed up. 3) verified strong vacuum from vacuum hose going to Booster. 4) removed hose from booster and verified air sucked in.

So, here's the kicker. When completing a hard brake, engine dies, in neutral or drive. To me, I think it would be a vacuum leak causing the engine to die. I think the vacuum leak might be from the brake booster, despite it passing all the tests. I've never seen a brake booster front plate (where the master cylinder mounts to) flex when the brakes are applied.

Any input would be greatly appreciated.

Thanks!

in2pro

Welcome to the forum...

It sounds like a booster issue to me... on my previous 95 Tahoe I had an issue of needing to press really hard on the pedal to get the vehicle to stop and it was the booster, though I replaced both booster and master cylinder at the same time.

warthogcrewchief

I can understand, if the brake was hard to press...it would be a tell-tale sign that it was a booster. However, the brake is not hard to press. Despite the ops-checks I've performed, indicating the booster is good, the engine dying sounds just like a vacuum leak from the booster.

At $130-$150 for a new booster, I want to troubleshoot it correctly. If I replace the booster, I'll most likely get a new master cylinder as they're only $30.


I wonder if the booster is on the verge of complete failure - works in park, but when braking under a load, it fails.

It still does not lead to why the brakes are so soft, unless a faulty booster can cause a master cylinder to fail.

It's perplexing...

warthogcrewchief

Thank you for the welcome! Sorry for not being more amiable/polite last night...I'm really focused on this problem & solution.

I'm waiting to hear from one of my family friend's this morning who has been a mechanic for over 30 years. Like In2pro said, I also suspect the booster is bad...but at that expense, I want a solid "second opinion".

Thanks again for the reply.

in2pro

No problem at all I don't blame you, can only diagnose so much over the web...
hopefully you will get more input from others over the weekend as well...
Keep us posted on what you find, it could really help someone else down the line...

warthogcrewchief

Will do. I may be new to this forum, but not new to forums or vehicle maintenance. My experience is mainly in aircraft, motorcycles, and automobiles. Although, as I just bought my '97 Suburban, so I'm learning about the specifics to the systems on this SUV.

I installed new rear shoes, rear wheel cylinders, and brake hardware. I bled the rear brakes. While cleaning the rear brake parts, I found the self-adjusters (both sides) were frozen. I had to use two pairs of pliers to break them free to clean and lube. I pumped the brakes again, and no better.

I then replaced the master cylinder (after bench bleeding the master cylinder). After install, I bleed it again to ensure it was good. I then bled all four wheels. I think this puts me at four times for each wheel this weekend!

Now, it was a little better, but not perfect. The pedal will still go to the floor. I hear an odd sound (maybe chattering), when the ABS activates. From my aviation background, my thought is it might be air trapped somewhere in the ABS system.

When I slam on the brakes, the brake warning light on the dash illuminates. It doesn't always stay lit. On a positive note, the engine doesn't die when I slam on the brakes.


I don't see any fluid leaking anywhere to indicate a ruptured line. My only other thought would be a brake booster or something in the ABS system.

Any input??


EDIT: I was doing some research. Apparently there has been an issue with the mid to late 90's Chevy trucks/SUVs ABS sensors corroding. I'm thinking I'll pull the ABS fuse for troubleshooting. If it works better, I'll then pull the sensors and try and clean them. At least that might solve one issue. Still trying to figure out why I have no brake pressure.

warthogcrewchief

In2Pro,
You called it. The part that finally helped was a vacuum booster. Turns out the master cylinder did fail and leak into the vacuum booster. I changed out the master cylinder last night and the vacuum booster today. I also changed out the ABS module because I had a nasty sound coming from my ABS sensor area when the ABS engaged. I looked on craigslist for trucks that were being parted out (around here all salvage yards are closed on Sundays). I paid $75, which I think is a fair price.

The parts I've changed out:

front pads
rear shoes + hardware
rear wheel cylinders (both sides)
master cylinder
vacuum booster
ABS module
enough brake bleeds to loose count

Thanks for the advice...I should have listened to you.