Hard pedal, no boost - good vacuum, good servo, good MC. - Chevelle Tech
Brakes, Suspension & Steering Conversion questions & more.

 
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post #1 of 6 (permalink) Old Jul 10th, 13, 7:43 PM Thread Starter
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Marek
 
Join Date: Jun 2011
Location: Warsaw, Poland
Posts: 34
Hard pedal, no boost - good vacuum, good servo, good MC.

Hello,
I have a problem with hard pedal and no boost on my breaks. It used to work just fine, but then, suddenly, when I pressed break pedal, started up engine, I heard some noise (something breaking, like balloon poping in from of the car) and there was no boost anymore. I need to press pedal very hard to break now.

Why it is not a vacuum problem:
- I did use two different valves (one brand new)
- I did change the hose to new one
- I did check with vacuum meter - 12" (0.04MPa)
- I did put long hose and connected my vacuum to other car (Capriece with almost identical break system) - everything is fine
- I did put long hose to other car, and connected to my booster - no boost

Why it is not servo problem:
- I used another, brand new servo - no boost
- I used same new servo in another car - working fine

Why it is not MC:
- I used MC from another car - no boost.

So I used my vacuum in other car, other car's vacuum in mine, I have used two different servos (one of them checked in another car), I used two different MCs in all possible combinations and... no effect.

Any clues? I've read something about stuck wheel cylinders, but my friend mechanic says it gives totally different effect.
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post #2 of 6 (permalink) Old Jul 11th, 13, 2:21 PM
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Dave
 
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Re: Hard pedal, no boost - good vacuum, good servo, good MC.

Puzzling. You have the classic symtoms of a bad booster. You never mention the check valve on booster. Worth testing. Start the car and let it run for a minute. Turn it off, then pull check valve out of booster. You should get a "whoosh" as vacuum escapes.
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post #3 of 6 (permalink) Old Jul 11th, 13, 2:27 PM
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Andy
 
Join Date: Apr 2010
Location: West Palm Florida
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Re: Hard pedal, no boost - good vacuum, good servo, good MC.

Marek, you have a bad booster.

When in doubt, get in the car. Pump the pedal 5-6 times and hold the pedal to the floor. Start the car. if the pedal goes completely to the floor, the booster is good. No change? Booster is no bueno.

Couple of junkers and a few clunkers.
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post #4 of 6 (permalink) Old Jul 11th, 13, 4:14 PM Thread Starter
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Marek
 
Join Date: Jun 2011
Location: Warsaw, Poland
Posts: 34
Re: Hard pedal, no boost - good vacuum, good servo, good MC.

Thanks guys, but that's not it.
I checked two different boosters, one of them is brand new, and works just fine in another car. Two different valves, both working good in another car. That has to be something else.

On second thought, pedal isn't actually hard from the top. It gets to about half of distance, and then stops rock hard. So it actually behaves something like in the car without booster at all while engine is running.
Pumping with engine off makes it hard, starting engine makes it go down to half of distance.

Could it be something with proportion valve? I'm out of ideas, and almost ready to rebuild breaks from scratch... Problem is I do have access to some spare parts, but not all of them (proportion valve being one of them) and it takes almost two weeks to get them to Poland. If I order something, which I'm not sure that will help, I may end up with wasting two weeks, and double price for that part (if you add shipping, and customs it's something like double price).
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post #5 of 6 (permalink) Old Jul 11th, 13, 4:18 PM
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Andy
 
Join Date: Apr 2010
Location: West Palm Florida
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Re: Hard pedal, no boost - good vacuum, good servo, good MC.

Yeah, if it stops halfway down it id trucly a mechanical issue. Noted that you tested, swapped, and puled a few hairs out, def look at the prop valve. It does have one moving part. Question #22, do you have decent fluid flow at all for corners?

Couple of junkers and a few clunkers.
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post #6 of 6 (permalink) Old Jul 19th, 13, 8:09 AM Thread Starter
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Marek
 
Join Date: Jun 2011
Location: Warsaw, Poland
Posts: 34
Re: Hard pedal, no boost - good vacuum, good servo, good MC.

So I double checked everything, and finally disasembled booster. It was ok, whit air pressure valve just a bit dirty. I assembled it again, and... It's much better now. Kill me, but I don't have a clue what was the reason.
Anyway, now I get very nice break boost until midium breaking. When I try to break hard it gets worse. I suspect low vacuum level to be the reason (11"). I'll try tomorrow to run engine, increase ignition timing and idle to get more then 16" and I'll see how it works. If it will be better, I'll have to get electric vacuum pump (I found used one here from Audi A4 for less than $25. Would it be fine?)

I still wonder why it did work better, then it broke up. In the mean time I did lower idle as it was pretty high, so it's propably the reason I can't get previous boost level now.
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