BRAKE experts needed....non F-body
01-09-2011, 03:58 PM
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BRAKE experts needed....non F-body I don't get the kind of knowledgeable responses like I do on LS1tech so since most brake systems work the same I'm hoping someone can help. My wife's 04 PT-cruiser's brakes are acting very strange...they are spongy and I can't get the wheels to lock up at all (at 40mph I stood on the pedal and it just slowed to a stop...didn't lock up and took double or more the distance than normal). The faster you go the longer it seems to get it to stop. I checked for leaks and everything is bone dry...no codes stored and its happened fairly quickly...within a week.
-engine off: two pumps on the pedal and its rock solid...holding it down and turning the engine on and its sucked down by the vacuum in the booster so I'm assuming that is working.
-Engine running: Once the engine is on it feels like the top 50% of the pedal does nothing...like its full of air. The lower 50% is a firmer but the pedal can be pushed down to the max with slight effort. Pumping the pedal while running does not firm them up no matter how many times they are pressed.
I bled all the brakes with a little hand vacuum pump...made no difference. All the rotors look good and there's 50% or more on all pads. I'm having a hard time deciding what to replace because Ive never had a problem in the upper part of brake system. I assume the master cylinder is not leaking internally or it wouldn't get rock solid with the motor off and even if the booster is bad I should still have been able to stop it by standing on it. Or maybe that's the problem I don't know....If the booster goes out could I physically push it as hard as the booster could?? I don't think that's it though because I'm bottoming out the pedal....I couldn't push it any harder when its on the floor...what about the proportioning valve? If that goes out what are the symptoms? Any help appreciated!
-engine off: two pumps on the pedal and its rock solid...holding it down and turning the engine on and its sucked down by the vacuum in the booster so I'm assuming that is working.
-Engine running: Once the engine is on it feels like the top 50% of the pedal does nothing...like its full of air. The lower 50% is a firmer but the pedal can be pushed down to the max with slight effort. Pumping the pedal while running does not firm them up no matter how many times they are pressed.
I bled all the brakes with a little hand vacuum pump...made no difference. All the rotors look good and there's 50% or more on all pads. I'm having a hard time deciding what to replace because Ive never had a problem in the upper part of brake system. I assume the master cylinder is not leaking internally or it wouldn't get rock solid with the motor off and even if the booster is bad I should still have been able to stop it by standing on it. Or maybe that's the problem I don't know....If the booster goes out could I physically push it as hard as the booster could?? I don't think that's it though because I'm bottoming out the pedal....I couldn't push it any harder when its on the floor...what about the proportioning valve? If that goes out what are the symptoms? Any help appreciated!
01-09-2011, 04:18 PM
#2
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Bleed them manually with an assistant in the driver seat. Crack the line open and have the assistant slowly depress the pedal to the floor. Make sure its down, then close the line and repeat. RR,LR,RF,LF in that order. If it doesn't tighten up, you may have a defect inside the master cylinder.
01-09-2011, 07:24 PM
#3
it's either air in the system or a bad master cylinder.
if the brakes have been serviced then there's a chance there is air in the system and that's the problem. But if the brakes haven't been touched and this problem just started happening then it's most likely internal leakage in the master cylinder and it needs replacing.
if it is air in the system, and if you're sure the air is at the calipers then you should be able to manually bleed them and get the air out. If there's any chance air entered the system at the master cylinder and made it's way into the abs modulator then you'll probably need a pressure bleeder attached to the master reservoir that'll push fluid through the system to remove the air, manually pumping the pedal may not flow enough fluid to effectively pass the air down the line and out the caliper. Other than that you can either spend the money and buy a brake line pressure test kit and see what psi you're getting at each caliper or have a brake shop do it. Not much else to check from your description, you say there's no leaks and you're booster is working according to your description. With the abs modulator I would think you would hear and feel it activating, you could try disconnecting the electrical connection to it- you'll get the ABS light on but that would rule out the modulator and the wheels should skid, if they don't then you're not getting enough brake pressure at the caliper because of either air or a bad master.
if the brakes have been serviced then there's a chance there is air in the system and that's the problem. But if the brakes haven't been touched and this problem just started happening then it's most likely internal leakage in the master cylinder and it needs replacing.
if it is air in the system, and if you're sure the air is at the calipers then you should be able to manually bleed them and get the air out. If there's any chance air entered the system at the master cylinder and made it's way into the abs modulator then you'll probably need a pressure bleeder attached to the master reservoir that'll push fluid through the system to remove the air, manually pumping the pedal may not flow enough fluid to effectively pass the air down the line and out the caliper. Other than that you can either spend the money and buy a brake line pressure test kit and see what psi you're getting at each caliper or have a brake shop do it. Not much else to check from your description, you say there's no leaks and you're booster is working according to your description. With the abs modulator I would think you would hear and feel it activating, you could try disconnecting the electrical connection to it- you'll get the ABS light on but that would rule out the modulator and the wheels should skid, if they don't then you're not getting enough brake pressure at the caliper because of either air or a bad master.
01-10-2011, 08:12 AM
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Thanks for the replies...I'll try bleeding it with my wife and not the vacuum pump first and if it doesnt work I'll try the master cylinder. Im just curious as to why with the engine off does the pedal get rock hard. Two pumps and it only goes down about 20% before I can't push it any farther.
01-10-2011, 08:15 AM
#6
I agree, I think you have an issue with the Master. Do you change the oil on her car? I would make sure nobody put power steering fluid in the master resevoir on accident. That will wipe it out in no time. The way to tell is to pull the cap off and if the seal on the cap is all swelled up then the system is contaminated with something.
My dad used to have quite a few of the 3.8L cars like the bonnevilles, impala's, buick lesabre, etc. come in because of "spongy" brakes. Well, the PS resevoir is way in the back of the engine so people would accidentally put P.S. fluid in the Master cylinder and contaminate the system. He even saw oil change shops do it
Just a thought if you don't service your own vehicles.
My dad used to have quite a few of the 3.8L cars like the bonnevilles, impala's, buick lesabre, etc. come in because of "spongy" brakes. Well, the PS resevoir is way in the back of the engine so people would accidentally put P.S. fluid in the Master cylinder and contaminate the system. He even saw oil change shops do it
Just a thought if you don't service your own vehicles.
01-10-2011, 08:43 AM
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01-10-2011, 10:07 AM
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Thanks for the replies...I'll try bleeding it with my wife and not the vacuum pump first and if it doesnt work I'll try the master cylinder. Im just curious as to why with the engine off does the pedal get rock hard. Two pumps and it only goes down about 20% before I can't push it any farther.
because you don't have the advantage of atmospheric pressure.
you're foot/leg strength is typically estimated at 100 lbs, so with engine off the master cylinder is only seeing 100 lbs of input force which is not enough to cause the leakage in the master, so the pedal pressure gets firm and you think everything is fine. Of course you know without the advantage of atmospheric pressure if you ever tried to brake the car with the engine off or no brake booster vacuum it's very very hard even if the car is just rolling 5 mph.
When the engine is running and you have vacuum within the brake booster, you're 100 lb input force is amplified, I forget by how much, and then the fluid pressures that are happening within the master cylinder are enough to where there's leakage and reduced line pressure to the calipers.
01-10-2011, 11:27 AM
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because you don't have the advantage of atmospheric pressure.
you're foot/leg strength is typically estimated at 100 lbs, so with engine off the master cylinder is only seeing 100 lbs of input force which is not enough to cause the leakage in the master, so the pedal pressure gets firm and you think everything is fine. Of course you know without the advantage of atmospheric pressure if you ever tried to brake the car with the engine off or no brake booster vacuum it's very very hard even if the car is just rolling 5 mph.
When the engine is running and you have vacuum within the brake booster, you're 100 lb input force is amplified, I forget by how much, and then the fluid pressures that are happening within the master cylinder are enough to where there's leakage and reduced line pressure to the calipers.
you're foot/leg strength is typically estimated at 100 lbs, so with engine off the master cylinder is only seeing 100 lbs of input force which is not enough to cause the leakage in the master, so the pedal pressure gets firm and you think everything is fine. Of course you know without the advantage of atmospheric pressure if you ever tried to brake the car with the engine off or no brake booster vacuum it's very very hard even if the car is just rolling 5 mph.
When the engine is running and you have vacuum within the brake booster, you're 100 lb input force is amplified, I forget by how much, and then the fluid pressures that are happening within the master cylinder are enough to where there's leakage and reduced line pressure to the calipers.
01-12-2011, 08:19 PM
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A lot of cars today must include a service bleed of the ABS unit in addition to a manual bleed. Can be done with some of the high end scan tools as well as manufacturer specific scan tools.
