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Strange brake problem
Ivan - 30/10/15 at 07:44 AM

Have picked up a strange one (to me anyway) problem on my Cobra.

First some relevant background info:

383 Chevy Motor - hot cam.
Jag XJ6 Brake system
Willwood front Calipers
TH400 Automatic Gearbox with standard torque converter.

The problem:

When you brake on a trailing throttle or when idling brakes work well and pedal is normal.

When standing still or there is some throttle applied as in left foot braking or revving it up against the torque converter brake pedal goes very soft and brakes don't work so well.

Question:

Why would brakes go soft when there is very low vacuum? My first thought is that the one way valve in the vacuum line isn't working but why would lack of vacuum cause the pedal to go soft?

jossey - 30/10/15 at 07:52 AM

Wouldn't the bleed out from the vacuum cause the pedal to be hard?

Ivan - 30/10/15 at 07:57 AM

quote:
Originally posted by jossey
Wouldn't the bleed out from the vacuum cause the pedal to be hard?


Exactly - that's what makes it strange - it's doing the exact opposite to what one would expect.

jossey - 30/10/15 at 08:11 AM

Very strange. I have no answer as you have already ruled out what I thought the problem would be.

I guess it is the vacuum booster or master cylinder playing up but can't work out why it would act like your saying. I had an issue like this years ago and replaced Both and solved it but no mechanic could tell me what it was.

Dingz - 30/10/15 at 05:12 PM

Could it be the non return valve in the servo line has died?

britishtrident - 30/10/15 at 06:06 PM

Vac leak + non-return valve