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Old 04-02-2013, 11:40 AM   #1
dah97765
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Default Low brake pressure when car is ON

Hey everyone,

This is also a general question...

I replaced all of the brake hardware on my car (both drum brakes and hardware, both front brakes and one of the calipers), bled the brakes, etc etc. When the car is OFF the car has great brake pressure, but once I turn the car on, the brake pressure drops 90%

I can brake, but I need to drop it to the floor (which has zero resistance) to have any semblance of braking power.

I have been reading that this is either a master cylinder / brake booster issue. Could it just be the valve?

The more I read, the more I think that it is a MC issue. Can anyone confirm / contest any of this before I start buying parts / working?

Thanks!
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Old 04-02-2013, 11:43 AM   #2
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Default Re: Low brake pressure when car is ON

Did this issue exist before you replaced all those parts?
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Old 04-02-2013, 11:49 AM   #3
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Default Re: Low brake pressure when car is ON

No, but the brakes were "chugging" for lack of a better term - stuttering, making really bad mechanical noises. When I looked at the drum brakes, the cylinders looked like they were leaking pretty badly, so I replaced them, and decided to refresh the brakes in general and replaced all of the hardware at that point.
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Old 04-02-2013, 11:54 AM   #4
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Default Re: Low brake pressure when car is ON

If you didn't have a spongy pedal before the work was done, there must be a problem with the work you did. How did you bleed the system? Did you follow richpin's youtube video?
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Old 04-02-2013, 12:01 PM   #5
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Default Re: Low brake pressure when car is ON

yep, followed the video, though its possible I let the fluid get too low in the master cylinder while I was doing it. I took it to pep boys to have it 'professionally bled' and they came back saying that the booster was bad, but the symptoms don't seem to be lining up with that assessment.

EDIT - took it to pep boys bc I was hoping they would be able to diagnose it / eliminate the possibility of it being a bleeding problem, since I thought I may have let air into the MC
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Old 04-02-2013, 12:03 PM   #6
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Default Re: Low brake pressure when car is ON

Did you adjust the rear shoes?
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Old 04-02-2013, 12:05 PM   #7
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Default Re: Low brake pressure when car is ON

Quote:
Originally Posted by madpogue View Post
Did you adjust the rear shoes?
Yup, replaced them, along with all of the other hardware. They were both pretty tight up against the drum
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Old 04-02-2013, 12:08 PM   #8
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Default Re: Low brake pressure when car is ON

Because you thought you brought air into the MC, did you attempt to bleed the MC? If the the booster was good before the repairs, I would doubt it being bad.
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Old 04-02-2013, 12:17 PM   #9
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Default Re: Low brake pressure when car is ON

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Originally Posted by lamping.ap View Post
Because you thought you brought air into the MC, did you attempt to bleed the MC? If the the booster was good before the repairs, I would doubt it being bad.
I didn't - thats why I brought it to pep boys because it was cheap, and I specifically asked if they bled the master cylinder. They said yes, but I am starting to think that they didnt. I wanted to take it to a shop because it was cheap and I had never bled a MC before (though looking at some of the videos online, it seems fairly straight forward)

Agreed - I don't actually think its the booster - I think its the MC (whether it just needs to be bled (if pep boys didnt do it), or replaced). I wanted to mention the booster in case anyone on here had another opinion on it.
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Old 04-02-2013, 12:24 PM   #10
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Default Re: Low brake pressure when car is ON

If I take the MC off of the car and bleed it on the bench, do I need to bleed the brakes at the bleeding screws on the wheels again?
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Old 04-02-2013, 12:28 PM   #11
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Default Re: Low brake pressure when car is ON

I don't remeber or have the FSM in front of me as I'm at work, but I know the procedure is in the FSM as part of the bleeding process. I've never had to do that portion of it though. I'd try that, or bring a pair of safety glasses and tell the shop you'd like to watch, if the car is still there.
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Old 04-02-2013, 12:33 PM   #12
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Default Re: Low brake pressure when car is ON

Quote:
Originally Posted by dah97765 View Post
If I take the MC off of the car and bleed it on the bench, do I need to bleed the brakes at the bleeding screws on the wheels again?
If you disconnect the lines that goto the wheels, your answer will always be yes. IIRC, The procedure in the FSM doesn't require bench bleeding, just disconnecting the lines. Maybe someone else here can post it, or I can in a hour or so.
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Old 04-02-2013, 12:55 PM   #13
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Default Re: Low brake pressure when car is ON

Quote:
Originally Posted by lamping.ap View Post
If you disconnect the lines that goto the wheels, your answer will always be yes. IIRC, The procedure in the FSM doesn't require bench bleeding, just disconnecting the lines. Maybe someone else here can post it, or I can in a hour or so.
Okay. My assumption was yes, but I wanted to make sure I wasn't doing work for nothing.
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Old 04-02-2013, 12:56 PM   #14
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Default Re: Low brake pressure when car is ON

And if you could post that, it would be awesome
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Old 04-02-2013, 01:41 PM   #15
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Default Re: Low brake pressure when car is ON

Two related questions: When you intially bled the brakes, did you have an assistant pushing the brake pedal while you cracked the bleeders (manual method)? And if so, did you put a block of wood or something similar on the floor of the car to limit the stroke of the brake pedal during bleeding?

If the answers are yes and no in that order, you may have killed the master cylinder by stroking the seals into the crusty, unused portion of the master cylinder bore. Soft pedal is amplified by the vacuum boost present when the engine is running.
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Old 04-02-2013, 02:14 PM   #16
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Default Re: Low brake pressure when car is ON

Here is the procedure. This requires two people.

http://www.scribd.com/doc/133656034/...Brake-Bleeding
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Old 04-02-2013, 03:06 PM   #17
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Default Re: Low brake pressure when car is ON

1-Do you have ABS? Having ABS may make some shops 'fearful' (aka ignorant) of touching master cylinders. Only a well trained person familiar with many anti-lock brake systems will be aware of where to bleed correctly. Its easy to assume a shop knows what they're doing until its found otherwise. A reputable shop specializing in brake systems would be the 'go to' place if not sure of who to trust. Plain brakes, before ABS/traction control/electronic stability control, allowed almost anyone with a good idea of hydraulics to flush/bleed brake systems. With ABS/TC/ESC, this can be troublesome to all but the very well trained person or anyone capable of DIY work with familiarity in hydraulics, electronics/vacuum brake boost units and all the interactions that makes plain brakes a thing of the past. Unfamiliarity with ABS/TC/ESC interaction with basic hydraulic brakes can be a recipe for disaster - a safety issue meddling with things that are assumed to work but don't when finding out the wrong way.

2-Without ABS/TC, brake systems are relatively easy to work on. The ABS unit is plumbed into the brake system similar to a 'T' fitting in all four brake lines to allow ABS to take over immediately when brakes are jammed for all they're worth as ABS runs the brake system, not the driver in a panic stop. The foot massage is the only tell tale indicator of ABS taking over braking duties while the driver is along for the ride..............

Without ABS, master cylinder bleeding is straight forward, right at the two output fittings. The m/c can be bled on the car without having to be removed for bench bleeding. With ABS, certain precautions are needed to know where to bleed the m/c - directly off the two fittings or on the ABS unit where two fittings are provided for this. Service manuals are needed to know specifics. It would be presumed that repair shops are already aware of the many ABS/TC/ESC systems out there and use good judgement. Here's where it can get complicated and troublesome. Who knows what and are capable of performing correct brake flushing/bleeding procedures with ABS/TC/ESC???????????????

Any mistake with flushing/bleeding procedures with ABS/TC/ESC can create more problems as well as a safety issue (poor to non-existent braking). As far as my limited knowledge goes, Saturn ABS/TC systems are OFF when the engine is not running (the only correct method for flushing/bleeding) to allow flushing/bleeding procedures. OFF meaning none of the ABS valves are open to allow air ingestion that would render ABS useless and ineffective when operating at the moment its needed. Flushing/bleeding brakes is straight forward with or without ABS.

3-The vacuum operated brake boost unit provides the 'power' to power brakes. It fails in one of two ways; no vacuum from the engine or meddling with the push rod that's part of the brake pedal to master cylinder rod. The brake boost unit itself has never been reported to fail - most likely because of its simplicity and only needing vacuum to power it. The other way to disable it would be from meddling with the push rod that's used in the master cylinder. Losing power assisted brakes will be felt immediately as the car won't stop and require all your worth to press the brake pedal down.

The brake test to separate a brake booster unit from interfering with brake hydraulics/flushing/bleeding procedures is braking with the engine OFF (always). Normal pedal feel will take place after pressing the brake pedal several times to bleed off the vacuum reserve in the brake boost unit until the pedal firms up - pedal travel is less than 3-4 inches and feels firm, not spongy. This firm feeling in the pedal is the actual brake hydraulics working without power assist from the brake booster unit. This is what its supposed to feel like long before power brakes and ABS became a standard feature. True braking to feel connected to the brakes. Sizing of the master cylinder doesn't allow real braking effort (pressing down for life or death) to brake our cars so power assisted braking reduced the size of m/c's to take into account the reliability of power assist to provide the extra power to help lock up our brakes (before ABS).

The difference between a firm brake pedal (engine off) without sinking when standing on the pedal and a pedal sinking to the floor with engine on can be attributed to; air in the system, misadjusted rear brakes using drums, leaking fluid, very old brake lines, damage to the ABS unit allowing brake fluid past any of the many valves used to isolate each brake, and a worn out master cylinder. Its very easy to question what failed but another to troubleshoot and separate/isolate pedal-to-the-floor issues. Its also easy to poo-poo the idea or periodic fluid replacement but no one explains the exact cause of pedal sinking relative to having brakes that are power assisted along with ABS having more valves in the brake system than ever - more parts to corrode while ignoring periodic fluid replacement.

When the engine's idling and brakes applied, the soft pedal is the result of power assistance from the vacuum boost unit multiplying leg power effort - more leg power more boost to lock up brakes (without ABS). If the pedal falls to the the floor and all flushing/bleeding efforts have been done correctly then the problem boils down to; old brake lines near each wheel, worn out m/c, possible valve leak in the ABS unit. When power assisted brakes are used, more hydraulic pressure is generated than understood. The higher pressures generated will get past worn seals. The m/c is one place bit no one asks about all the valves used in ABS units...............
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Old 04-19-2013, 11:32 AM   #18
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Default Re: Low brake pressure when car is ON

Bleeding the two fittings on the MC worked like a charm... my suspicions of pep boys not actually bleeding it were confirmed. Managed to convince them that I shouldn't pay for a cent of that operation.

Thanks, everyone, for your contributions and helping me figure this out. One more little nugget of knowledge tucked away.

Now for my leaking coolant... haha.
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Old 04-19-2013, 12:19 PM   #19
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Default Re: Low brake pressure when car is ON

So PepBoys in your local area failed to perform needed brake procedures when asked? Which PepBoys? Posting the specific one may help others avoid this one. This also serves as an open forum to warn any businesses looking in to be aware of a national/international forum that poor business practices isn't acceptable and there's a way to publicize this dissatisfaction.
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Old 04-19-2013, 06:25 PM   #20
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Default Re: Low brake pressure when car is ON

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Originally Posted by fdryer View Post
So PepBoys in your local area failed to perform needed brake procedures when asked? Which PepBoys? Posting the specific one may help others avoid this one. This also serves as an open forum to warn any businesses looking in to be aware of a national/international forum that poor business practices isn't acceptable and there's a way to publicize this dissatisfaction.
Local pep boys at 41st and Market st in Philadelphia, PA 19104.

The manager eventually caved and removed the charges, but was not happy about it and only did so after multiple iterations of trying to placate me.

I'm very jaded about the situation, especially considering after I explained the issue and specifically asking about bleeding the MC, that was not done...and they came back with ~$500 worth of parts and service to 'fix' the issue.
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