The V8 Owners Forum -

Hi everyone, hopefully someone can shed some light on a problem I have.
I own a Dax Cobra with a Rover 4l V8 and lt77 box, the engine was rebuilt a few months ago before I bought the car. The issue I have is this......
When driving along I push down the clutch pedal and give a little rev (in tunnels as I think most of us would do) the car noticeably slows as if the brakes are being applied. It’s more apparent at certain speeds and in certain gears. I thought is it a vacuum thing, the brakes are servo assisted. Then waiting at the traffic lights the car was slowly rolling forwards, I gave the accelerator a little push with the clutch pedal down and the car stopped moving. I’m no mechanic but I know that this should not be happening as I have never experienced this before in any car I’ve driven. I have noticed that the car is not particularly lively and feels like it’s being held back. The car was rolling road tested and is delivering 182 bhp. So what is the problem and what is the cure?
Your thoughts please.

Colin.

Does anything get hot, like a single wheel? If the car is dragging somewhere, (assuming that the power is there), then something ought to be getting hot.

Chris.

Wondering if a fault in the servo or whatever could cause the brakes to come on by themselves? Be inclined to jack the car up and get some to rev the engine etc and see if it is happening by turning a wheel by hand.

Nothing gets hot that shouldn’t and there’s no binding, car rolls freely when not running. All I can think of is when the clutch is disengaged and I rev the engine it creates a vacuum in the servo that applies the brakes. But how could this happen?

Could the whole pedal box be moving and applying the brakes with the clutch?

There’s no problem with the pedal box, if I drive and press the clutch and don’t touch the accelerator then the problem isn’t there.

Have you tried clamping the vacuum pipe, Or removing and blocking it, perhaps mole grips on the pipe.and take it for a drive and see if the same happens with the servo not connected/functioning?

Be careful though as you will need good leg muscles to stop it.

If it is anything like the GT40, I removed the servos and adjusted master sizes to get excellent brakes with no servo needed.

Ian

Thanks for the input, I will try blocking the servo in the next few days and give an update.
Ta.

Ralphypants wrote: ↑

Fri Jun 19, 2020 3:12 pm

All I can think of is when the clutch is disengaged and I rev the engine it creates a vacuum in the servo that applies the brakes. But how could this happen?

It couldnt. And most vacuum is generated on the overrun, or idle, or cruise, etc etc. Not blipping the throttle.

I really dont think driving the car without the servo working is a good idea.

Another thought.

Are the pedals mounted on a common shaft!
If so are they dragging on that so when one moves (accelerator) the next pedal ( brake) is also moving?

Ian

Update. I’ve had a very good look at this now, yes the pedals are mounted on a common shaft and yes the brake pedal does move and it moves more the harder you press the accelerator. It’s very slight so I never noticed it. Odd thing is that there is a spring connected to the above bulkhead that was hooked onto the brake pedal in order to pull it up. Therefore this was an existing problem that was kind of bodged. With the spring reconnected, well it helps a bit but it is no cure. Come to think of it there was always a bit of a whistle from the rear inboard brakes when I was on throttle and vanished when not. Therefore I have concluded that when I’m pressing the accelerator the brakes have been operating all the time
So, what is the cure? New pedal box or tougher spring?

Colin.

Well at least you have the cause.

What would I do. In order I would go

Oil the bearing surfaces, WD40 may work.

A second or stronger spring may work.

Pin the shaft so it cannot twist meaning all the pedals have to move on the shaft.

Replace pedal box

Replace pedal box and reconfigure master sizes and remove servos. This would depend on your brake sizes and if enough hydraulic pressure could be generated.

Ian

Well my pedal box does have a bit of character but it’s not the main cause of the problem. I blanked of the vacuum hose went for a test drive and the problem has gone. I have good brakes that I actually prefer as I have a VW buggy without a servo. The car pulls better, sounds better, rolls freeer. The servo must have been operating the brake slightly when ever the accelerator was pressed. I will look into the cause as some point but for now I’m of for a drive in the sun.
Thanks to all those that took the time to think about my issue, you’ve been very helpful

Colin.