When I start my car the brakes engage themselves until they’re fully engaged I’ve replaced the brake booster 2 front calipers and brake lines but for some reason without fail it happens everytime

 

A little history, please. Is this a new problem out of the blue? What was the initial condition which prompted you to change all those parts? Was some other fault present, and then this new problem started after a part was replaced? Or were all of those parts changed in hopes of fixing this new issue?

My first thought is a problem with the adjustment of the brake booster pushrod. I've no experience with the Jaguar booster, but on all others I've touched, you have to adjust the pushrod that actuates the master cylinder. Most replacements include a little jig to set it properly during installation.

 

So originally I thought my calipers or brake hoses were either seized or clogged when i would turn on my car and apply pressure to the brakes the pedal wouldn’t return so I replaced the calipers & brake hose but no difference so next was was the booster and it worked yes I had to take it apart but I used the old push rod from the old booster so I knew there was exactly a .5mm gap and the brakes work great I’ve got great pedal to stop ratio it’s just as soon as I start the car the front calipers engage and if I crack the bleeder they release and gradually engage again until fully engaged I’m not sure if it’s a computer thing or if the brake fluid is wrong I’m not sure because too me somethings not adding up

 

Hmm, this is a strange one.

I wonder if the master cylinder piston is not fully retracting, for reasons unknown. Maybe a swollen seal or something like that.

The booster sees this errant position and thinks you have applied the brakes, so it applies extra force as designed. This is all guesswork on my part, so don’t take it as gospel.

I’m not very familiar with the ABS and DSC systems. Is either system capable of applying pressure on their own? Or do they strictly reduce individual braking action under certain conditions? I thought the latter, but don’t really know.

To differentiate which system (booster, ABS, or DSC) is at fault, try this: Park on a level surface and chock the wheels. Disconnect and plug the large vacuum line to the booster. Ignition off, cycle the brake pedal at least 3 times to deplete any residual vacuum in the booster. This will totally take the booster out of the equation.

Now turn the engine on and see what happens. (Don’t drive like this, as excess pedal pressure will be required to stop.) If the brakes still apply themselves, it would have to be ABS or DSC doing it. If nothing, it had been caused by the booster.

If the booster was acting up, the next step would be to determine why. Is the booster itself fine but is out of adjustment somehow? Is it acting as designed if the MC piston is not fully retracted? Is the new booster defective? Sorry I don’t have a definitive answer for you.

 

You took the booster apart?


Is .5mm the correct spec? That sounds a bit small to me.

I would go with what Karl has mentioned.

Or maybe just slightly loosen the master cylinder mounting nuts and see if anything changes?

Cheers
DD

 

I loosened the master cylinder bolts to check if the issue was the push rod this but it is not and can the seals swell from just the wrong brake fluid?

 

Yes, wrong fluid can swell the seals.

To clarify, very wrong fluid can swell the seals. Like using power steering fluid rather than brake fluid.

I'm not sure if using the wrong type of brake fluid would swell the seals. The spec is to use Super Dot 4. Would Dot 4, or Dot 3, swell the seals? I dunno; I don't think so.

Let's hear what others have to say

Cheers
DD

Cheers
DD

 

Does the brake pedal move when this is happening? The pedal probably has a return spring, so maybe you'd have to apply very light pressure to overcome it. If the pedal doesn't move on its own, press very gently to see how much free travel the pedal has. You're not trying to apply the brakes, just taking up slack. Compare to the free travel with the engine off and everything normal.

Just thinking out loud, trying to determine if the booster is moving. If the free travel increases with the engine on, this means the booster is moving.

 

Pressure of the pedal is really good & when I turn the car on the pedal drops

 

Can you please elaborate how you measured the gap? Did you use the special tool for the task? Some other means to set the gap?

So far the clues are primarily pointing towards a problem with the pushrod adjustment. It would be a good idea to revisit how it was accomplished.