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Old 09-05-2006, 11:51 PM
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Combination valve !@#$%^&*

Since I have owned my '69 Impala (5yrs) I've gone through about 7 master cylinders. Every master cylinder I get leaks around the lid/leaks out the back onto the fire wall or just plain fails. Last fall the brakes went out again...I got another mc as usual, bench bled it, installed it, flushed a whole new quart of fluid through the brake system, bled the wheel cylinders. Pedal was nice and firm afterwards-went around the block and pedal went to the floor the 3rd press. Tried bleeding them again, same thing happened. Replaced all four wheel cylinders. Still no pedal. I am at a loss. It is a 4 wheel drum setup, NO power assist. Pretty simple you'd think. At this point Im thinking the problem lies in the combination valve, but it seems replacements aren't available. I just seems like air is getting in somewhere. Any ideas?

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Old 09-06-2006, 12:23 AM
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Are you buying all these master cylinders from the same place? Is it a reputable supplier?
What are you using for a pushrod to bench bleed your mc. Could you be scratching the cylinder bore?

The combination valve would not allow your pedal to go to the floor unless it was leaking profusely.

Does your pedal adjustment allow your master cylinder piston to return all the way?

Are your brakes adjusted properly?

Questions, questions, questions

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Old 09-06-2006, 12:52 AM
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No, the first replacement was a used one. Lasted like a year. Second-brand new, third-remanufactured, were Autozone specials. Forth, fifth, sixth and seventh are from Advance Auto-remanufactured. All replaced under warranty.
I am using a wooden dowl to bench bleed them.
The pedal does allow the piston to return all the way, infact there is about 1/8" play before the pedal makes contact with the mc piston. Is that a problem?
Shoes are new too. Adjusted so that the drums just slightly drag on them.
There is absolutly no fluid loss. I just lose pedal pressure after about the third or forth application. And only when applying the brakes while moving. I can pump the pedal all day long in the driveway-feels fine. Drive down the street, hit the brakes, pedal gets mushy. Accelerate, stop again, gets worse. Third time, gotta pump the crap out of the pedal to stop.

Last edited by godsMistake; 09-06-2006 at 01:17 AM.
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Old 09-06-2006, 01:13 PM
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Check the rubber sections at the front wheels of your brake lines for balooning.
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Old 09-06-2006, 01:50 PM
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Do the brakes work ok the first time after you bleed them? How are you bleeding the brakes?

Is your combination valve centered?
If you get fluid from all the wheel cylinders when you bleed it it should be centered..

when you bleed the system after it experiences the loss of pedal pressure, is there air in the system?

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Old 09-06-2006, 01:51 PM
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If It Were Mine...

I would source a quality master cylinder, and no they cannot be found in discount auto stores.

You said a take-off lasted a year and there are multi-failures with the rebuilds. Make sure the application is drum/drum and they are not sliding you another application.

There should not be a combination valve on this application. You may have a pressure differential valve (triggers light on loss of brakes).

Brake hose ballooning is very possible as suggested. When you get pressure back, have a helper watch all three brake hoses. Maybe if they are ballooning, the M/C is over extending itself (piston) leading to failure?
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Old 09-06-2006, 02:28 PM
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I know the mc's are correct, as I researched the various types used; drum/drum, disc/drum, power/non power, thinking that they were giving me the wrong one.
I will double check all the hoses, but they were replaced about 2yrs ago.
As far as bleeding them, I'm just doing the old pump the pedal, open the bleeder. Starting from the farthest rear wheel to the closest front.
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Old 09-06-2006, 02:38 PM
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Quote:
Originally Posted by powerrodsmike

Is your combination valve centered?

If you get fluid from all the wheel cylinders when you bleed it it should be centered..

...when you bleed the system after it experiences the loss of pedal pressure, is there air in the system?

Quote:
Originally Posted by godsMistake

As far as bleeding them, I'm just doing the old pump the pedal, open the bleeder. Starting from the farthest rear wheel to the closest front.
Have you checked the system for air after failure is what he is asking. This would determine air ingestion or internal M/C failure.

Your brake warning lamp works OK?
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Old 09-06-2006, 03:01 PM
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Thanks Kultulz, air ingestion was why I was asking that .

So then you get good flow from your wheel cylinders when you are bleeding them and your brake warning light is off and your pressure differential valve is centered?

Does the system ever return to normal operation with out bleeding?
Like if you drive in reverse and apply the brakes, will the mushy pedal go away?

can you get the system to work now? Have you bled it again since you experienced the loss of pedal pressure?

Did the brakes work ok before you replaced the M/C?

Does the pedal sink slowly to the floor when you pump up the brakes and apply steady pressure to them or will it maintain a pedal height?

You are sure there are no leaks at the bleeders or lines?

later, mikey
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