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Discussion Starter #1
I have noticed the revs drop a little when pressing the brakes
For example when coming to almost a stop instead of the engine settling to 900 rpm it will dive down to 800 or less and then pick up again. Feels like it will stall at times

Is this a problem with the vacuum pipe to the servo ?
 

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Hi,

It's probably not related to the pipe, it would affect the engine operation permanently (by leaning the mixture).
But a small air leak within the servo drum may cause this symptom -> outside air enters through the servo drum,
when applying the brakes. Does the RPM drop / irregularity stays until you keep the brake pedal depressed?
 

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Discussion Starter #4
hmm I'm entering uncharted territory here.

Lonewolf: If this drum has a small air leak is there anyway of testing this in the workshop? Not an easy thing to replace here (or cheap).
The revs drop initially, but it stabilizes even with the brake pressed.
So maybe not this then? Could it be a carb issue - floats at wrong height? (think I read somewhere if you brake suddenly then revs can drop)


rowdy79: what symptoms did you have with the one way valve failing? where did you get a new one?
 

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Discussion Starter #5 (Edited)
Ian from the other alfa forum told me some time ago if the revs drop when you brake hard it can be because the floats are set too low.

Another possibility?

one test for the servo drum would be this I guess:

if the diaphragm in your booster is bad, which means that applying the brake exposes the engine to a major vacuum leak (it's a huge hose) and messes up the mixture, causing it to run rough? A check for that would be to start the engine with the brake pedal depressed - if the pedal gets softer after the engine starts, your booster is working right
 

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Discussion Starter #6
ok so here's a list of possibilities:

1) servo brake drum torn - air leaking
2) excess fuel entering - inlet valves leaking (have to check to see if I can see fuel dripping down barrels)
3) servo one way valve leaking?
4) floats too low - fuel level drops too much when braking hard

idle is fine
cracks and pops on deceleration (indicating a nice lean mixture and not rich though)

car will go in for a brake service soon (along with new 16v oil pump fitting once that arrives), so I can get the servo checked there by the pros.
If it comes back with the same issue then guess it will be the carbs.
 

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I agree with the test method mentioned by Ian.

The revs drop initially, but it stabilizes even with the brake pressed.
So maybe not this then? Could it be a carb issue - floats at wrong height? (think I read somewhere if you brake suddenly then revs can drop)
At sudden braking with maximum brake force, all atmospheric air from the servo drum
will be drawn into the engine. This leans the mixture for a short period of time.
On LHD 33s the vacuum for the servo is taken from the intake port of the 4th cylinder,
so the mixture balance of this cylinder will be affected. So we can say that, a short period
of irregular idle is normal when applying the brake suddenly.

If this drum has a small air leak is there anyway of testing this in the workshop? Not an easy thing to replace here (or cheap).
Yes, the diaphragm leak could be tested, even at home. You'll need:
- a vacuum meter (I guess you're already have one)
- a "T" fitting for the meter, which is suitable to insert into the brake servo line
- a valve (maybe a fish tank valve will be OK)
- a vacuum pump could be useful, but the engine vacuum could be used as well

To test both one-way valve and the diaphragm at once: insert the valve and the
"T" fitting (with the meter) between the engine (or vacuum pump) and the inlet of the one-way valve.
The valve should face to the vacuum source.
Start the engine (or the pump) and wait until the vacuum reaches -0.6 psi (or more,
up to -1 psi), then close the valve, and shut down the engine.
Check the vacuum drop on the meter. If the depression stays more than an hour,
then IMHO the valve and the diaphragm inside the servo drum are OK (try to avoid any
leaks, when the extra hoses and fittings are connected, it could give misleading results).
 

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Discussion Starter #8
I agree with the test method mentioned by Ian.


At sudden braking with maximum brake force, all atmospheric air from the servo drum
will be drawn into the engine. This leans the mixture for a short period of time.
On LHD 33s the vacuum for the servo is taken from the intake port of the 4th cylinder,
so the mixture balance of this cylinder will be affected. So we can say that, a short period
of irregular idle is normal when applying the brake suddenly.



Yes, the diaphragm leak could be tested, even at home. You'll need:
- a vacuum meter (I guess you're already have one)
- a "T" fitting for the meter, which is suitable to insert into the brake servo line
- a valve (maybe a fish tank valve will be OK)
- a vacuum pump could be useful, but the engine vacuum could be used as well

To test both one-way valve and the diaphragm at once: insert the valve and the
"T" fitting (with the meter) between the engine (or vacuum pump) and the inlet of the one-way valve.
The valve should face to the vacuum source.
Start the engine (or the pump) and wait until the vacuum reaches -0.6 psi (or more,
up to -1 psi), then close the valve, and shut down the engine.
Check the vacuum drop on the meter. If the depression stays more than an hour,
then IMHO the valve and the diaphragm inside the servo drum are OK (try to avoid any
leaks, when the extra hoses and fittings are connected, it could give misleading results).
thanks for that advice Lonewolf.
Not sure where I'd get a t fitting for the large diameter vacuum hose though. Yep got a couple of vacuum meters and fish tank valves.
issue is trying to get that T fitting to fit the small diameter meter and large vacuum house.
good trick though :thumbs:
 

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Discussion Starter #9
Checked out the carbs today.
Totally dry while idling. No drips whatsover

Checked the tightness on all clips.
Drove and still the Same

So next step is to get the servo drum checked.
Brake fluid needs changing anyway.
Suspecting the servo now
 

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Discussion Starter #10
when we talk about the servo drum we are referring to the diaphragm inside right? :eek:
Nothing related with the master cylinder no?
 

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Discussion Starter #12
Thanks man.
How does the one way valve work?
It's open when the brakes are not in use and when you press the brake the valve closes?

Maybe worth trying to get another one to see if this is faulty
 

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Generally a one-way valve permits gas (or fluid) flow in one direction only.
These valves are operated by the pressure difference present between
the input and output sides of the valve.

The purpose of this particular one-way valve is to maintain the depression in the servo drum
(as low as possible), under various throttle conditions, or even at (unexpected) engine stall.
Under certain conditions the vacuum present at the intake port could be lower (more close to
the atmospheric pressure) than the vacuum inside the drum. The valve won't permit the equalization
of the pressure difference in this case.

So the valve is open when the depression in the intake port is higher than the
depression in the drum. The valve operation is not affected by the braking directly.

There's an arrow on the valve housing, which shows the permitted air direction.
If the valve permits flow when the air direction is reversed, then the valve is faulty.
 

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Discussion Starter #14 (Edited)
If the valve permits flow when the air direction is reversed, then the valve is faulty.
thanks for that great explanation :thumbs:

So if the valve is leaking a little, under braking conditions this could be pulling a small vacuum from the right head causing the revs to drop? (the valve should snap close when braking)
:)

If I need a replacement I guess most will do as long as they have the same pipe diameter.
something like this:
http://www.ebay.co.uk/itm/Brake-servo-brake-booster-non-return-valve-CAC4188-CAC4188A-/281139749604?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item41753c22e4

I will try and find one locally to rule out that possibility. Lot cheaper than the servo!
 

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I'm glad if you've found it useful.

So if the valve is leaking a little, under braking conditions this could be pulling a small vacuum from the right head causing the revs to drop? (the valve should snap close when braking)
If the valve is leaky (not towards the outside air - that's a different story -,
just between the input and output), then the servo efficiency will directly depend
on the engine vacuum, and the brake pedal force may vary.
But it won't pull air from the intake, because the depression values of the drum
and the intake will be nearly the same (with a small hysteresis) in this case.
The valve fault will allow pressure equalization, so the vacuum inside the drum will
follow the intake vacuum.
 

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Discussion Starter #16 (Edited)
I'm glad if you've found it useful.



If the valve is leaky (not towards the outside air - that's a different story -,
just between the input and output), then the servo efficiency will directly depend
on the engine vacuum, and the brake pedal force may vary.
But it won't pull air from the intake, because the depression values of the drum
and the intake will be nearly the same (with a small hysteresis) in this case.
The valve fault will allow pressure equalization, so the vacuum inside the drum will
follow the intake vacuum.
:confused:
so in other words it cannot be this one way valve causing the revs to drop?

:rolleyes:

cheers

one more thing: the brake fluid is low, just a couple of mm's below MIN. It's going into the workshop to have a change in the next couple of weeks so I thought it wasn't worth topping it up.

THIS has nothing to do with the issue does it?
 

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IMHO failure of the one-way valve (in itself) not causes RPM drop at braking.

If there's no sign of brake fluid leak at the clutch slave cylinder, or any other visible parts
of the brake system, there might be a minor leak towards (into) the servo drum.
If you decide to remove the brake master cylinder, in order to check the leak on the back,
a new sealing ring will be needed between the drum and the MC during reassembling, to
provide air-tight sealing of the drum chamber.
 

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Discussion Starter #18 (Edited)
IMHO failure of the one-way valve (in itself) not causes RPM drop at braking.

If there's no sign of brake fluid leak at the clutch slave cylinder, or any other visible parts
of the brake system, there might be a minor leak towards (into) the servo drum.
If you decide to remove the brake master cylinder, in order to check the leak on the back,
a new sealing ring will be needed between the drum and the MC during reassembling, to
provide air-tight sealing of the drum chamber.
thanks.

Low level of fluid won't have any effects on the revs dropping while braking no?

no sign of exterior leaks. could be leaking inside as you mention.
I won't touch the master or servo. job for the workshop 'specialists'.
will try and book it in for a couple of weeks from now.
I will check the pipes leading from the servo to valve - head again.

shame because all this was replaced some 5 years ago before I purchased it I believe. all looks new.
 

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Low level of fluid won't have any effects on the braking no?
Until the air not gets into the cylinder, the braking effect won't be affected.

Well, if the master cylinder was replaced 5 years ago, the possibility of the
cylinder leak is very low.
 

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Discussion Starter #20
Until the air not gets into the cylinder, the braking effect won't be affected.

Well, if the master cylinder was replaced 5 years ago, the possibility of the
cylinder leak is very low.

I'll top it up just in case. Want to rule out all possible causes before a mechanic starts stripping the lot!

Bit of DOT 4 won't do any harm no?
 
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