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#338485 - 08/04/06 10:37 AM Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both?
Expired Offline
10+ Year
Member

Registered: 03/30/01
Posts: 1089
Loc: Anytown, USA
in connection with another post...

Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both? See my other "..slow speed brake..." post.

The original problem is that I have to put the pedal as far down as it will go and then the car will slowly roll to a stop afterwards at slow speeds. I wont drive it anymore, im afraid...


With the engine off I can pump the brakes and the pedal will "pump up". When I turn on the engine, the pedal will drop. This would tell me that the booster still works, correct? I replaced both check valves(one that goes directly into the booster, and the "in-line filter" looking thing. The line attaches to the back of the Edelbrock Carb no leaks, clamped where it could be done.

I bled all of the brakes, everything is fine there, fluid is clear in reservoir now. The front brakes are brand spanking new, completely. I thought this was the problem before. The back brakes are fine as well as the rubber hose, no leaks anywhere.

The master cylinder was replaced in 1999 with a Cardone piece from Advance Auto Parts. The car has been stored 90% of its life since 2001. The brake light does not come on either.

Can anyone tell me how to test the master cylinder or booster to tell which one I need? I don't want to have to replace one or the other if I don't need to.

Please help!!

Josh

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#338486 - 08/04/06 11:23 AM Re: Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both?
Chevrolord Offline
Member

Registered: 09/30/01
Posts: 2613
Loc: West Virginia
Sounds like your booster is ok, the pedal falling away on startup indicates that. Does the idle change at all when you step on the pedal?

Are the rear brakes adjusted up well? If you set parking brake, then hit the pedal, does it feel different?

Are the drums true and have proper thickness? Is there a lip at the edge of the drum surface?

Do you have a firm pedal, or a lot of travel?

edit- Here ya go:

Test for a system vacuum leak as described below:

* Operate the engine at idle without touching the brake pedal for at least one minute.

* Turn off the engine, and wait one minute.

* Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. Light application will produce less and less pedal travel, if vacuum is present. If there is no vacuum, air is leaking into the system somewhere.


Test for system operation as follows:

* Pump the brake pedal (with engine off) until the supply vacuum is entirely gone.

* Put a light, steady pressure on the pedal.

* Start the engine, and operate it at idle. If the system is operating, the brake pedal should fall toward the floor if constant pressure is maintained on the pedal.

One thing I have came across when a power brake booster goes bad, sometimes when applying the brakes the engine rpm will go up. What in effect is happening is, the booster is causing a giant vacuum leak, thus when you depress the pedal it lets the engine suck air. Of course the above tests should indicate this, but if you have noticed this when driving you may want to do these tests to check it.

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#338487 - 08/04/06 08:15 PM Re: Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both?
Expired Offline
10+ Year
Member

Registered: 03/30/01
Posts: 1089
Loc: Anytown, USA
Based off of what you have posted..in my case the booster is fine. The idle does not go up when I push on the brake pedal. The rear brakes are fine. I had to wiggle the drums off of the shoes but both sides looked fine, adjusters were fine, yes it was rusty, but I sprayed a bunch of brake clean off of it and cleaned everything up the best I could. The brakes bled really well also. I haven't tried setting the parking brake and then pushing the pedal to feel a difference, I will give it a shot and let you know.
After tying it a few more times now I think I have alot of travel vs a firmer pedal, it feels firm towards the end of the travel and then I can feel it hit as far as the pedal will go. I am leaning towards a master cylinder but another person with the same symptoms tried that and it didn't solve his problem, hes trying a new proportining valve. Have to see what he finds out from that....

Appreciate all of the help!

-Josh

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#338488 - 11/09/06 04:47 PM Re: Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both?
2manymontees Offline
Member

Registered: 10/12/05
Posts: 1294
Loc: Whitmore Lake, MI
I have the same exact problem. I've been through 3 master cylinders. The braking is great for about 3,000 miles and then slowly gets worse., This is the first time its been bad enough for the brake light to go on. I installed another new master cylinder today, and it did not get better this time, which leads me to believe that there are other problems. Or it could be just another crappy master cylinder from Autozone. Could the reservoir not be sealed tightly enough? It is the original.

I am going to pull the master cylinder off of my SS which was working properly. Just was hoping that there is an easy solution,.

Booster is good. And all of the bleeders open up, the rear brakes are tight and the pads are all new. There are no leaks anywhere. Could a proportioning valve be the culpret?
_________________________
1980 Monte, Vapor Grey/Claret, 229 V6 (LC3)/TH-350/2.41, 101k. (in resto.)
1983 Monte SS, White, 305 (L69)/TH-350C/3.42, 39k.<<2012/13 NMCOA Class Winner
1984 Monte SS, Med. Blue Met., 350 V8 (ZZ4)/TH-700R4/3.73 POSI, 76k. (Father/Son Car)
1987 Monte LS, Lt. Blue/Dk. Blue, CL interior, 305 (LG4), TH-2004r, 2.41, 176k (weekend driver)

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#338489 - 11/09/06 04:56 PM Re: Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both?
Expired Offline
10+ Year
Member

Registered: 03/30/01
Posts: 1089
Loc: Anytown, USA
hmmm, not sure. If the brake light comes on then you have a pressure loss if I understand it(and if you have that you should have a leak). What is the condition of your hard brake lines? I bought a new/not reman master cylinder from Advance Auto Parts and it went bad after two weeks, the replacement works great now...light never came on though. The proportioning valve might be bad, but that is usually rare. I replaced nearly every brake part but that and the brakes are fine now. I believe my problem was a combination of the M/C and Booster.

Let me know what you find out,

Josh

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#338490 - 11/09/06 05:48 PM Re: Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both?
2manymontees Offline
Member

Registered: 10/12/05
Posts: 1294
Loc: Whitmore Lake, MI
Nope no leaks, brake light only goes on for a moment and then if I let off and pump the brakes it goes away. Breaks feel spongy overall, but they have been bled to death, not much more I can do. With the new master cylinder it "feels" the same and the pedal has A LOT of play and goes nearly to the floor before the car stops. It takes forever to stop. I am going to pull my working master cylinder off my other monte as I sure hope that the "new" autozone master cylinder is no good. I am on my 4th one!! This is nuts. This one I think was bad out of the box. but only replacing it with another will tell.
_________________________
1980 Monte, Vapor Grey/Claret, 229 V6 (LC3)/TH-350/2.41, 101k. (in resto.)
1983 Monte SS, White, 305 (L69)/TH-350C/3.42, 39k.<<2012/13 NMCOA Class Winner
1984 Monte SS, Med. Blue Met., 350 V8 (ZZ4)/TH-700R4/3.73 POSI, 76k. (Father/Son Car)
1987 Monte LS, Lt. Blue/Dk. Blue, CL interior, 305 (LG4), TH-2004r, 2.41, 176k (weekend driver)

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#338491 - 11/10/06 08:57 PM Re: Bad Master Cylinder or Bad Brake Booster? How can I tell w/o buying both?
2manymontees Offline
Member

Registered: 10/12/05
Posts: 1294
Loc: Whitmore Lake, MI
ITs a BAD master cylinder, the 3rd one from AutoZone... I put the old one off my SS on it in the pouring rain!! Stops on a dime..
_________________________
1980 Monte, Vapor Grey/Claret, 229 V6 (LC3)/TH-350/2.41, 101k. (in resto.)
1983 Monte SS, White, 305 (L69)/TH-350C/3.42, 39k.<<2012/13 NMCOA Class Winner
1984 Monte SS, Med. Blue Met., 350 V8 (ZZ4)/TH-700R4/3.73 POSI, 76k. (Father/Son Car)
1987 Monte LS, Lt. Blue/Dk. Blue, CL interior, 305 (LG4), TH-2004r, 2.41, 176k (weekend driver)

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