Thread: Weird brake pedal behaviour

Ozdunc,

Two completely different opinions here but that's why it is called a forum.
your call...

Phil.

Alright, I ran the test to find out whether the MC is pulsating.

With vac plugged, I drove up and down the road, brakes were nice and smooth no pulsing or grabbing.
Vac is 21-22inHg at idle with a nice steady needle.

Then just for kicks I reconnected the vac and did an emergency stop from about 20km/h - it practically stood on its nose.

So either is my dodgy driving - I don't think I was jiggling my foot on the brake, more like slight pressure, quick increase in pressure, back to slight pressure, quick push, back to slight, etc without releasing the pedal fully. (Although to be fair up until the pedal went hard I wasn't concentrating on the brake system, but merrily chatting away to the missus. Even then I hid my surprise well, don't want to give excuses to take her car out on errands )
Or its something in the booster. My booster looks different to the example you gave above Dave, in that there don't appear to be any mechanical pushrods, just fluid in/out with what looks like an additional MC attached to the booster


From the top

from the bottom in front of the firewall.

I presume this design would still have a valve and poppets, but would it be more liking to be leaking fluid if there was a diaphragm/rubber seal failure?

That is a remote VH44 type booster the same as I have on my series 3.
That may account for the issue but I'm sure Dave will give us more advise.

Phil

I'm valuing all the opinions.

If just turns out to be my driving, I've learnt an absolute heap about testing brake boosters.

And if it turns out be the booster/vac, I'm a better driver than my massively inflated ego already thinks I am.

Cheers

I couldn't agree more !!
I am exactly the same boat.

Phil.

The remote booster works exactly the same as the explanation Dave gave, except that instead of a pushrod you have a column of hydraulic fluid from the master cylinder to the booster.

It still comes down to your having either a usage problem or a lack of vacuum, either due to a leak, a defect in the booster, or a restriction in the vacuum line. It also occurs to me that you would get a reduced vacuum if you have the engine idling too fast.

John

Thanks John,

With that, the results of Dave's initial tests and the fact they're working great now I'm going to take on a blow to my ego and assume dodgy driving technique.

If it happens again I'll do the rolling hill tests and go from there, at least knowing the booster model means I can get parts or get it refurbed if necessary.

Thanks everyone, esp Dave, for your help.

Hi, I think u will find it was your driving style that caused the problem with the brakes. Try fitting a remote vacuum reservoir into the system. it will give you a greater capacity of vacuum for the braking system. You can hide it under or behind the left side front guard or above the left footwell area. Cheers Rod