Ok, I need ideas. I think it's the brake booster, but I'm not sure (never killed one before, LOL!)

My Cavalier is a 2000 with a 3.8L supercharged V6 (L67) engine in it. I'm using the stock Cavalier master cylinder and brake booster, hooked up to the same line the Park Ave the engine came from was using... vacuum normally not a problem. But... most of the time, if I'm in reverse... and some of the time, if I do a hard turn (like pulling a yoo-ee!)... the brake pedal goes right to the floor.

The car stops... albeit slowly.... because the booster isn't providing any boost, the pedal goes to the floor... then the brakes do kick in... but it's a little disheartening when it suddenly happens... especially in reverse!!! That's the part I don't get...

Any ideas??

...j

John,
I'm shocked!!! Usually you are the one that people go to for the answers. I need to mark this day on the calender.
Serious note: I can only assume you have checked the obvious items since you take care of your vehicle the same way I do.
I will still ask, have you checked for possible clogged lines? Have you insured that there is no air in the system? These are the
first couple of things that come to mind.



98 Z24

RIP Specks

Hey, I don't know everything... and I'm wrong sometimes too!! Every now and then I come across something I don't know.. so I search the JBO... and I usually find the answer... but this one has me kinda stumped.

Yes, I've checked the lines.. clean as a whistle (where did that expression come from?), bled perfectly so there's no air... nice clean new fluid... nothing is leaking...

Since I'm using the Park Ave 12" front brakes, and the rear are 11" discs from an Alero... maybe this booster isn't of a large enough capacity for the system? It just strikes me as weird that it only happens if I'm turning or in reverse.... not regular driving (ever!). I can do a hard quick stop and there are no problems... but if I do a three-point turn... the pedal drops to the floor.

...j

John I was just pulling ya leg.

I am not a master of the Cavi's either, but I try to think through things logically. If this is only happening during hard turns or in reverse
I'm just wondering if something isn't preventing the proper fluid pressure during extreme turns and reverse? What about maybe the ABS
sensor on the front wheels? Hey guy, just trying to throw some ideas your way!!!



98 Z24

RIP Specks

I hear ya... I appreciate the ideas Which is exactly why I made the post, so I can figure out what I'm forgetting to check!! ha ha...

ABS is disabled at the moment.. so not that. I had no idea how to diagnose the master cylinder or the vacuum booster... need to sort those two out. Maybe I need a larger booster? I have the master cylinder from the Park Ave, and was planning on putting that on over the winter (so the ABS and tire pressure monitor will work properly!). Perhaps this will fix or at least change the problem...??

...j

With the way things are going, you do that and ya might not have any breaks. HA HA HA!! That would bite!!
I have a call in to my son-in law who took a course on brakes and has changed out mine and my daughter's
quite a few times. Hopefully, he might know what could be the problem. Email me with what kind of setup you are running and I'll
pass the info onto him.


98 Z24

RIP Specks

John don't worry about emailing me. I pieced together what you have in the car. If I get a response from the son-in-law, I'll let ya know.


98 Z24

RIP Specks

Never heard of that but would think it is the booster. Did this start suddenly, or has it been happening for some time?



FU Tuning

If the booster does not have any vac. in it you wont have any brake assist ie. very hard pedal. I totaled my 00 from a faulty booster, I had no brakes. With 2 feet on the pedal I could not press it down enough to stop the car within a normally reasonable distance.



Edited 1 time(s). Last edited Sunday, October 12, 2008 4:25 PM

Hello John, Long time since we chatted, I got a few questions: When you apply the brakes, Does it feel like its got pressure sometimes, feels faded like overheated brakes? ( Like brake fade). If so thats 9/10 master cyl. Also brake booster usually u can feel bogg on idle with the brakes pressed. I could be totally wrong, Ive had a few issues with brakes messing up, Taken it to 2 different shops they said it was my calipers, Along it was my master cyl.

Good Luck.

does it almost seem like your hitting a wall with the braking? and do your vacuum fluctuate when it happens?

My $.02

How are your rubber brake lines?

I would test the mater cylinder by swapping it out with another J master cylinder.

Have you thought about the diaphram in the booster being ripped?

I've had my booster fail on me, would hit the brakes and it'd feel as if you hit a brick wall. Vacuum would fluctuate.

um, I agree with mitdr774 of whatever. If the booster is bad, you will have a HARD pedal, sounds like you are starting at the floor.....with no resistance.....which is NOT a booster problem.

Do you have ABS?

master cylinder! is bad or has something floating around in there thats gets in the way of the piston seal letting it leak past then pedal goes to the floor! ru sure that u r not loosing fluid?


see ya!

John Son-in-law finally got a hold of me. His theory is what has been stated to you from others. It sounds like the booster might not be strong enough since you swapped engines and are using the original booster. That is his theory. Ya might want to swap boosters.


98 Z24

RIP Specks

Thanks for all the responses and thanks to ZSleeper for reminding me about this thread

Car update... I didn't do anything with the car yet... I still find it odd that it always does it with the wheel cranked hard over, and usually in reverse... but never any other time.

The 'vert is currently parked for the winter... being a soft top, and it'll be pissing rain in Vancouver now til May or so... so over the winter I plan on changing the J-body master cylinder out for the C-body (Park Avenue from the donor car) master cylinder. That way, I'll have the tire pressure sensor system working... apparently it was based on the ABS sensors in that module year, not using the actual green sensors inside the tires like they use now. Looking forward to that

I'll post back in May when the car is on the road again!!

...j

I'm going to have to say that if the pedal effort is really hard or presses really easily, but the brakes hit hard and fast, are usually indicators of booster problems (not enough or no booster = hard to push but have brakes, too much booster = soft pedal, very firm brakes.) The power brake booster's sole purpose is to assist you pressing the brake pedal, so you don't have to press so hard.

Sounds more like air in the lines or fluid going past the seals in the master cylinder. If you press and hold the brakes for an extended period of time the pedal sinks to the floor, then it's the master cylinder. If you pump the pedal and it firms up, then your getting air in the system somewhere.

As for changing the master cylinder, you could end up with firmer pedal or softer pedal, depending upon what the master cylinder bore diameters are. If the cylinders are larger, you'll have less pedal travel, firmer pedal and they'll come-in faster. If the cylinders are smaller, then you have to push further, but not as hard and the bakes come in more slowly. this has to do with the master cylinder to caliper piston/wheel cylinder volume ratio. For example: the original master cylinder (A) has a bore of .75" (.442 sq in section area) is replaced with MC B with a 1.125" bore (.994 section area). With MC A you had to put 5 # of pressure on the pedal and move it 1.5" to stop, just changing to MC B you would only need 2.5 # of pedal pressure and .75" travel to stop. (this is only an example for reference, not any actual figures.) this is because MC B has approximately twice the sectional area as MC A, hence only half the travel and pressure are needed to enact the same results.

Most all production cars have one rod with two master cylinders(one in front of the other). This is done so if on cylinder/piston fails, you still have half the brakes. The whole system is layed out as illustrated below:

Pedal - Rod - Booster w/rod - Master cylider (Rear Piston w /Rod - Front Piston) - Lines (hard and soft) - each caliper or wheel cylinder.

Car is slowly being awakened from her winter slumber... I've changed the WW spoiler for a stock 03-05 one, so between that and some other minor things over the last couple of days, I'm just about ready to put insurance back on it...

I haven't changed anything with the brake system yet... I'll do a bleed first and see what happens, if I get any air out of the system. If things remain the same, then I'll be venturing into the C-body master cylinder swap and wiring changes to make it work... as I had originally planned. Haven't gotten that far yet, I've been far too lazy over the winter... plus we have a puppy now, so I've been somewhat, ummm, distracted??

...j