Strange brake problem

[Snake2998]

Have posted this on another forum as well - lets see who comes up with the answer:

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

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 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 when in theory it should go hard?

[Jaydee]

Sounds like air in the hydraulic lines. With the brake booster working at high vacuum you might not have noticed it.
JD

[DanEC]

I assume you have a vacuum assist brake booster. If the booster is working and holding vacuum properly, after running the engine and shutting it down, you should then be able to apply and hold the brakes 3 or 4 times before the pedal eventually goes soft and starts to sink towards the floor. If when you first apply the brakes and the pedal after a moment starts sinking to the floor the booster is not functioning properly.

And as stated your engine may just not produce enough vacuum with a hot cam for proper operation.

[Jaydee]

If you hold the pedal down and the pedal sinks to the floor, it might mean that the master cylinder seals are bypassing. Leaking internally.
JD

[joyridin']

This is probably a stupid question, but are you sure it is vacuum controlled? Sounds a bit more like what a hydraulic booster would do.

[Snake2998]

see post below

[Snake2998]

Thanks for the input all - you have given me some pointers to look at.

A couple of replies:

1 The pedal is rock solid as long as there is vacuum - it doesn't sink at all.
2 It is definitely a vacuum booster.
3 I haven't tried the pedal after switching off to see how long it takes to lose vacuum - i.e. to see if the one way valve is working.
4 The pedal only sinks when throttles open with engine running - I haven't tried it when in neutral - will do so next time I get to the car - it's currently 50km away from me.
5 The pedal is rock solid before starting the car after a week or so - haven't tried starting with pedal depressed to see if the pedal sinks a little as normal when vacuum is restored - will do so.
6. There's no apparent fluid loss in system as brake fluid reservoir is still full.

[Jaydee]

Another way to test it. Is step on the brake with engine of, pedal should be hard, then whilst on the brake ,start the engine, the pedal should go down a bit more, but feel a bit softer. Then with engine of you should get a couple of boost assists.
JD