OT: brake booster or master cylinder?

before i begin, the vehicle in question is a 2001 chevrolet impala.

the vehicle in question is my mother's. i drove it approximately 2 weeks ago and noticed that the brakes were extremely "grabby" and that the pedal was very stiff once one proceeded past the force needed to stop the car. looking at the master cyl/resivoir (sp)/booster yesterday, i noticed a moisture on my hand, having passed it under the cylinder, saw "dirt" indicative of a leak on the body of the cylinder itself. I topped the resivoir(sp) off about a month ago, and the level was still perfectly level with the full mark. i only had time to bleed one of the back calipers, which yielded little to no improvement. the brake pedal travels almost double the distance with the engine off than with it on. anyone want to wager a guess as to what it could be? thanks.

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Originally Posted by ryan roopnarine

before i begin, the vehicle in question is a 2001 chevrolet impala.

the vehicle in question is my mother's. i drove it approximately 2 weeks ago and noticed that the brakes were extremely "grabby" and that the pedal was very stiff once one proceeded past the force needed to stop the car. looking at the master cyl/reservoir (sp)/booster yesterday, i noticed a moisture on my hand, having passed it under the cylinder, saw "dirt" indicative of a leak on the body of the cylinder itself. I topped the reservoir(sp) off about a month ago, and the level was still perfectly level with the full mark. i only had time to bleed one of the back calipers, which yielded little to no improvement. the brake pedal travels almost double the distance with the engine off than with it on. anyone want to wager a guess as to what it could be? thanks.

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Only circumstantial advice... Bentley's has a test for the BMW booster, that reads something like: "pump the pedal 10 times with the engine off and hold it down. Start the car, and if the brake booster is working properly you should feel the pedal drop a bit." Sounds like the behavior you are seeing is the opposite?

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Originally Posted by Robin-535im

Only circumstantial advice... Bentley's has a test for the BMW booster, that reads something like: "pump the pedal 10 times with the engine off and hold it down. Start the car, and if the brake booster is working properly you should feel the pedal drop a bit." Sounds like the behavior you are seeing is the opposite?

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This sounds like a good test for most power-assisted brake systems, be they either hydraulic or vacuum assisted. The whole idea of the pumping is to remove all traces of power assist to see what happens upon starting the engine. If all works right in the power assist system, the pedal should drop due to the extra pressure applied by the assist system.

The above assumes a healthy brake hydraulic system. If the master cylinder is leaking internally (bleeding down) causing the cups to bottom out in their bores, then no amount of power assist will help the cups go deeper. A bad master cylinder can make you think the booster is bad so ensure the brake hydraulic system is good to go before condeming the booster.

Grabby brakes can be caused by oil on the pads/shoes or by a bad control valve in the booster. So the first and easiest/cheapest course is to have a look at all four corners to ensure there is no grease/oil problems or blued/overheated rotors/drums.

If the pads/discs look good to go, then bleed the hydraulic system. You are trying to get a tight pedal, that does not drop when held down, with the engine off. Once you are sure the brake hydraulic system is solid, then the only other choice is the booster.

Brian

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Originally Posted by Robin-535im

Only circumstantial advice... Bentley's has a test for the BMW booster, that reads something like: "pump the pedal 10 times with the engine off and hold it down. Start the car, and if the brake booster is working properly you should feel the pedal drop a bit." Sounds like the behavior you are seeing is the opposite?

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my pressing the pedal down with the engine on and off is a derivative of the bentley test that, for whatever reason, sticks in my mind whenever i'm playing with the brakes on any car---bee em troubleyou or not. this time, i just forgot to exhaust the assist before making the post! i went to the public library and copied the relevant section out of the MOTOR repair manual, as i assume that mitchell and whatnot would be the same. i was hoping that it could be something dumb like resevoir grommets, but i doubt that those would really make that much of a difference.

i'm going to bleed all four corners and inspect them at the same time, won't hurt anything to run new brake fluid through it. seeing the rebuild instructions for the master cylinder in the manual has gotten my cheap bone all frisky. some people are too ADD to be doing things like that......

thanks for clearing that up brian, i was a little unclear about some of the rationale behind some of the symptoms.

ahh, here is the verbatim text from the 2002 MOTOR manual about checking the master cyl for leakage, ergo, the bentley test is relevant and applicable to any GM car......

MASTER CYLINDER INTERNAL FLUID LEAKAGE CHECK
Start engine and depress the brake pedal. If the pedal gradually falls under constant pressure, the hydraulic system may be leaking. Raise the vehicle on a lift and check all tubing lines and backing plates for signs of leakage. It may be necessary to lift or remove the carpeting or floor mats to check for booster or master cylinder leakage.

A leaking master cyl will cause the pedal to get soft and the travel to increase. A bad booster causes a hard pedal.
Does the booster retain vacuum? Disconnect the check valve to see. While you're at it check the check valve for one way flow.
My guess is the booster but your mention of a possible leak would indicate the master. Are you sure there was/is a leak?

the amount of grime accumulation in/around the threads of the brake lines going into the master cylinder, coupled with the odorless clear liquid that wetted my hand after rubbing (it) around the MC make me think that it is leaking, though that might not be the case. like i said, i had topped it off a month ago, and discovered no fluid loss according to the side of the resevoir, but am seeing a tremendous amount of "mess" on the MC.

a quick swipe and smell of my hand indicates that there is brake fluid on the underside of the master cylinder. it would appear that the fluid has been running down the front of the booster for a while now. pumping the pedal to release the reserve and starting the engine yields a pedal that depresses once the engine starts, but the car still has grabby brakes. have not had a chance to look at each wheel for leakage, though.

ok, last post, i promise. i would assume, for the purposes of being thorough, that the brake fluid leaking from the reservoir was due to me refilling it last month. i bled all 4 courners yesterday. i didn't find any leaks, but found that though three of the wheels bled with average, almost booster like pressure on the brake pedal, the passenger rear (the furthest from the mc) took significant force on the pedal to bleed. it also, imo, pumped the most fluid per number of pumps compared to the other calipers. any thoughts? thanks.

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Originally Posted by ryan roopnarine

ok, last post, i promise. i would assume, for the purposes of being thorough, that the brake fluid leaking from the reservoir was due to me refilling it last month. i bled all 4 courners yesterday. i didn't find any leaks, but found that though three of the wheels bled with average, almost booster like pressure on the brake pedal, the passenger rear (the furthest from the mc) took significant force on the pedal to bleed. it also, imo, pumped the most fluid per number of pumps compared to the other calipers. any thoughts? thanks.

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So one wheel is not like the others. It is rather strange that the pumping force required was high, yet the volume of fluid was high also. I figure a restriction, like a stuck porportioning valve or an outright line/bleeder blockage would cause one wheel to pump 'harder' yet with a much reduced volume of fluid due to the restriction.

But you have high pressure and high volume. Maybe there exists an equalizer or 'balancer' valve that is sticking initially, and upon releasing, you get the 'gusher' out the bleeder hose. If said valve is sticking, this would help account for your 'grabby' brakes once the valve 'unstuck'.

I am not up on this car's brake system but I would say time spent checking any valves in the brake hydraulic circuit would be well spent.

Brian.