Brake pedal gets rock hard immediately off boost - vac pump going?

[ahtoxa]

Hey all.

So today, I was merging onto a freeway under boost in 4th (long ramp), and when I got off boost to gently tap the brakes to align myself with traffic, the pedal was rock hard for a brief moment. It went back to normal after a fraction of a second.

I can replicate this condition every time if I get on the brakes straight off boost. The longer I'm in boost, the worse it is.

So, reading online, I see that the likely culprit is the vac pump. What supports my thinking, is that I've had a weird intermittent buzzing heard while at idle. It sounded very much like a pump turning off and on repeatedly. So, I suspect that I either have a failing vac pump, or maybe I have a vac leak at the pump and brake booster.

Any thoughts or tips? Going to crawl under the car today and see what I can see. Hoping it's a simple vac leak that I can fix.


Sigh. Always something wrong with this car.

 

[ahtoxa]

Yeah, my vac pump is running all of the time, now, without building vacuum. Turn the ignition on and all I hear is buzzing from it.

Gonna crawl in there after work today and see what I see. Hoping that it's a simple issue like a loose line or what not.

 

[ChicksHateMe]

Sounds like the problem I had. Did you ever see my thread on the issue?

http://www.sonicownersforum.com/for...on-discussion/15407-my-brakes-scaring-me.html

Mine was a problem from purchase, but didn't show up for about 6 months (3,000 miles).

For me it was a bad check valve that let the booster fill with brake fluid.

Have you/they been filling your brake fluid more than usual?

I could duplicate it by accelerating and trying to press the brakes. Trying to use the brake while accelerating was a no go, they were hard as a rock. Once I let off the accelerator my brakes went from rock hard to useable again. What sucked for me was I was using cruise control and the car was accelerating at the time, and it scared the crap outta me. Nearly hit the car in front of me.

For Safety Sake, Try This;

1. Find a spot on the road with plenty of room in front of you to accelerate.
2. While Accelerating, press on the brakes.

Mine were rock hard if I did this.

I took it to the dealer two times to fix my issue;

1st time they replaced booster and master cylinder.

2nd time they finally replaced check valve and a vacuum pump.

I still wonder how much brake fluid made it past the bad check valve into the NEW booster because they didn't replace it the first time.

Sucky dealer left the brake fluid all over the place, Leaving me to clean their mess. At least the brakes seem back to new now, and it's been about a year.

Good luck with yours.

 

[ChicksHateMe]

Also, something I noticed the weeks leading up to my near disaster was a random, VERY high pitched squealing, and I couldn't really tell where it was coming from. No Pattern to it, and not specific length of time. Pretty sure now it was that check valve letting air past it, because it disappeared when that was replaced.

 

[ahtoxa]

Yeah, my symptoms are identical. Just during lunch, I was driving and left footing the brake. Rock hard pedal while I'm accelerating.

I have not yet checked the fluid level. Will do so after work today.

 

[ahtoxa]

To add, my symptoms are identical, with the exception that I can hear my booster pump running all the time. That tells me that it's trying to build vacuum but cannot, so I can only hope it's a simple fix such as a vac line.

 

[ahtoxa]

OK. So I climbed under and checked lines. Didn't see anything.

Checked line coming from the manifold through what I presume is a check valve and into the vac pump. Looked good.

Disconnected that line at the pump and plugged the hole with finger. Drawing vacuum but the pump still did not shut off when ignition is on.

Not sure if my finger plugging test was sufficient, but it seems that while pump is drawing vacuum.

What else should I check and how should I go about it? Tips would help.

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[jgrote]

Just clear things up, Chicks problem was that his MC was leaking brake fluid out of the back of it, and into the booster. That then allowed fluid into the check valve, which subsequently failed.

ahtoxa, If you are not losing brake fluid, (Are you losing fluid?) your problem is just in the vacuum side of things. The electric pump is there to supplement the system while under boost, but the engine creates vacuum most of the time. The booster just works as a huge chamber to make sure vacuum is available all the time. It sounds as if your electric pump is fighting a "leak" all the time. It may still be bad, but it isn't your only problem. I'm willing to bet that your check valve is not sealing properly, thus allowing your electric pump to suck air from the motor. Either that, or your booster has a leak in it.

Edit: the reason your finger test didn't work is that there is probably some pressure sensor in line that didn't see it hit target vacuum. The only way it would se it is if that sensor was in the pump itself. It's probably in the booster somewhere.

 

[ChicksHateMe]

Are you still under warranty?

I watched some good youtube video's on how to test the various parts, like the booster, the vacuum pump or check valves.

Probably better to avoid the crazy service center if you can do it yourself.

 

[jgrote]

Chicks, as you learned the hard way, even the dealer techs don't always know where to start. The problem with power braking systems is that one part goes bad, and subsequently ruins another part. If you don't fix all of the problems at once, you are just chasing your tail. Your issues were a hard pedal and missing fluid IIRC, your problems were a cascade of issues, one caused by the other.

For ahtoxa, they need to figure out where is vac is leaking first. Then they need to see what other damage is done. The duty cycle of the electric pump is probably something like 10%-30%. Since it's been running nonstop for who knows how long, it's probably damaged as well. I doubt the dealer will have this info, but there should be specs on that pump. It would look something like: it can pull 25in/hg in a 2 liter container in 20 seconds. To test the pump, you would hook the line to a container with a gauge mounted to it and see how long it took to suck it down.

(Note: all times and measurements above were made up arbitrarily. Don't use these for testing purposes.)

 

[ChicksHateMe]

jgrote,

Thanks for your post, That one helped me understand better how it started with the leak, that took out the Booster, which in turn took out the check valve.....

Hope ahtoxa can find and fix it fast cause that hard brake scared the crap outta me.

ahtoxa, make sure you avoid Cruise control With mine on when the problem first appeared I was about 1 or 2 car lengths away from a good mash up going 65mph.

 

[ahtoxa]

Jgrote - thanks. Your explanation of why my finger test failed makes sense.

No, I am not losing fluid, so vac leak is still my main hope.

The pump doesn't run all the time when engine runs, because then the vacuum is provided by engine. It only runs non stop when ignition is on but engine is not running.

I'll dig into it again and see what's up.

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[ahtoxa]

So I pulled the check valve off. It seems like there's a bit of dampness in there, but hard to say whether it's normal or not. I wonder what Chicks' valve looked like after fluid got in there, but mine didn't really look filled, if you will. Perhaps the dampness I saw was oil blowoff through the intake?

I found the GM part number for it online, which includes all the hoses. Pulling it off takes 20 second flat without removing anything to get to it or getting under the car.

I'm going to see if I can get one tomorrow at any of the local dealers or have them order it for me. If so, I'll swap it on right in the parking lot and see if it resolves my issue. If so - great and if not - I'll just return the part, I suppose.


This seems like the easiest and most logical first fix, so I'll try it and see if it works. If I were losing vacuum anywhere as jgrote suggested, it would be there.

 

[ahtoxa]

Also of note. My brake fluid is at about the half mark between max and min (but the reservoir isn't uniform, of course). I can't honestly remember where the level was before, but I'm pretty sure that it was never all that high and nowhere near the max. So, at first glance it doesn't appear that I'm losing fluid.

More so, I've had the vac pump come on and off for about 1k miles now or maybe more. I figure I'd have lost a ton of fluid now if it were leaking.

 

[ahtoxa]

Bad news.

Pulled the master brake cylinder off the vac booster. It's wet behind the master and some fluid ran down. Great.

So I'm in the same boat as Chicks.

Master leaked into booster and then the brake fluid ruined the check valve and possibly the pump is compromised.

I am pissed off. Off warranty now and **** won't stop breaking on this car.

I've had enough with this piece of garbage. Sure, I can fix it, but I have no more confidence in this car. It's rediculous that a car with 50k miles on it (all stock) has as many issues as mine had in that time period.

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[jgrote]

Master cylinder might be part of the powertrain warranty. Sorry for your luck man. This is 2 now; I wonder if this is going to be a common issue? Once again, it sucks when they can't figure out how to make something that has been in existence for 80-90 years now. The braking systems we have now are mechanically the same things we've had since the 1960s. Sure, ABS pumps and crap like that have become part of the equation now, but really the 2 hole master, brake calipers, and wheel cylinders haven't changed in for ever. These are not parts that should fail.

 

[ahtoxa]

At least not in 50k miles. But I guess it happens. Just seems to happen with my car quite a bit in general.

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[franl24fan]

Had mine go bad

At least not in 50k miles. But I guess it happens. Just seems to happen with my car quite a bit in general.

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I had this part go bad at about 12k miles, they replaced the master cylinder. I was told any future problems in the same system would be covered even if out of warranty. Right.

The brakes have always been an issue from day 1. Very grabby, no way to adjust them. I actually have a sign I put on the steering wheel if I have to let someone park my car.

Since I have such low mileage, about 26k, I expect more issues as time goes by.