Hard Brake Pedal when cold

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Hamish WAUGH - 30 Jan 2005 16:48 GMT

'91 Mazda 626 2.2 not turbo.

On especially cold mornings, when I first start out, the brake pedal
is hard as a brick and very high, like the vacuum assist isn't
working. It stays that way for 100 - 200 yards and two or three uses
of the brake, then it goes to normal. Vacuum connection from the
manifold seems secure. Am I gonna have to replace that whole stinking
vacuum assist unit? I hope this is not a dealer item. They will bend
me over the sink while some kid plays dueling banjoes.

Thanks for any help,

Hamish

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the fly - 30 Jan 2005 18:28 GMT

>'91 Mazda 626 2.2 not turbo.
>
[quoted text clipped - 9 lines]
>
>Hamish

The first thing to try is having the brake fluid replaced.
You may have enough water condensed in the cylinder and lines to form
a plug of ice. That makes for the hard pedal when cold.

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Don Bruder - 30 Jan 2005 19:40 GMT

> >'91 Mazda 626 2.2 not turbo.
> >
[quoted text clipped - 13 lines]
> You may have enough water condensed in the cylinder and lines to form
> a plug of ice. That makes for the hard pedal when cold.

Actually, with the other post about bad idle Hamish made, I'd have to
come to the conclusion that the vac. assist is the culprit for both
problems. (I'm also reading his use of the word "cold" to mean "engine
not hot/freshly started", as opposed to "temperature below freezing")

Hamish -
Try this test at a stoplight (or wherever you usually get the idle
problem: Stop, set the parking brake, GET OFF THE SERVICE BRAKE, and try
to duplicate the "screwy idle" - I think your other post said it happens
when you take the car out of gear. Can you duplicate it when your foot
isn't on the service brake? If not, that REALLY strongly points to the
vac. assist unit being your source of vac. leak.

Supposedly, the vac. assist is always under some level of vacuum (unless
the brakes have been applied often enough after engine shutdown to
equalize the pressure) but if you've got a pinhole or leaking check
valve, it could very well be "leaking down" overnight/while parked,
giving you the brake misbehavior you're seeing. Similarly, the "delay" I
think your other post mentioned between stopping and the onset of the
"weird idle" at a light could be caused by the same thing - At first,
vac. is normal, but as the brake is held, it leaks down to the point
where it needs to rebuild vaccuum, opens the valve needed to do that,
and effectively opens up a vacuum leak to the manifold, causing your
weird idle.

I *REALLY* think both problems are related to the same cause, and expect
that in the end, that cause will be found to be the brake vac. assist
unit.

A new vac. assist unit should be relatively cheap. "Should be" and
"relatively" being the worrisome concepts...

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Brent P - 01 Feb 2005 03:18 GMT

> Actually, with the other post about bad idle Hamish made, I'd have to
> come to the conclusion that the vac. assist is the culprit for both
> problems. (I'm also reading his use of the word "cold" to mean "engine
> not hot/freshly started", as opposed to "temperature below freezing")

My beater car, an '86 mazda had the same issue without screwy idle. Brake
pedal hard brakes less effective for the first few yards before going
away. I cannot offer a solution as the problem went away on it's own and
only happened on very cold mornings.

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Hamish WAUGH - 01 Feb 2005 13:18 GMT

Hi,

Thanks to all for responses.

Brake fluid was recently (less than six months ago) replaced.

I will clarify as to "cold." The brake problem only happens on
espectially cold mornings, under 20 F," And only first thing in the
morning, so the engine is cold when this happens.

The idle problem (see recap below) only happens when the car is warmed
up, and doesn't seem to be affected by outside temperature. It only
happens in gear, not in neutral, not in park, and, get this, NOT EVEN
IN REVERSE. The idle problem is the same whether or not I am using
the parking brake or the service brake.

This car has the dreaded "all-electric" transmission, which was
rebuilt about 6 months ago. I haven't dug around sufficiently to see
whether any vacuum lines go to it, but I have been given the
impression that none do. If I am wrong, and the trans. has some
vacuum input, is there any way that vacuum could leak only when the
car is in gear?

======
Idle recap.:

'91 Mazda 626 2.2 without turbo has a rolling miss at
idle when in gear. It runs and idles smoothly at all other times, but
if the car is fully warmed up and I stop at a traffic light, it will
idle smoothly for about four or five seconds, then the idle will drop
and surge, drop and surge, from almost stalled to normal. If I put it
back into gear it will go right to normal (about 750 - 800 rpm) and
stay there.
=====

Thanks for any help,

Hamish
==================================

>> >'91 Mazda 626 2.2 not turbo.
>> >
[quoted text clipped - 45 lines]
>A new vac. assist unit should be relatively cheap. "Should be" and
>"relatively" being the worrisome concepts...

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