Hi All,

Thanks for all the help so far!

The car T Reg 1999 S-Type V6 3.0 Auto.

Working though the brakes on this car. So far only investigation and no changes.
Symptoms. Pedal is hard (2-3 pushed to solid) when the engine is off., Turn the engine on and the pedal sinks to the floor. The brakes still apply.

Tests so far. results in blue
Taken from this thread https://www.jaguarforum.com/threads/s-type-2-7d-brake-problem.87639/#post-815584

Test 1

1. With the engine off, pump the brake pedal to remove any residual vacuum in the Servo. Pedal goes hard
2. Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the Servo is working properly. Failed, pedal moves to the floor

Test 2
Run the engine a couple of minutes.

1. Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight Servo. Failed Pedal does not get firmer

Test 3

1. Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the Servo is airtight. Failed, hard to test due to pedal movement

Inspect the Check Valve

1. Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the Servo. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced. Interesting... there is no check value, but servo does not move with pressure , but does with vacuum. see image below of lack of check valve and route of vacuum pipe.

Verify Vacuum

1. Check the operating vacuum pressure when the engine is at normal operating temperature. There should be a minimum of 18 in. of vacuum. Vacuum may be increased by properly tuning the engine, checking for vacuum leaks and blockages in vacuum lines. Passed 18in of pressure

The vacuum hose route looks interesting, I was expecting it to connect to the manifold at the rear of the engine, it does not, it routes to the front.


The route is clockwise in the images above. No check valve, I could add one in where the pipe meets the y join. Looking at the manual the check valve is where the hose meets the brake booster, not in line.



I am thinking that either the solenoid or master cylinder is at fault here.

I have not bled the brakes yet, I will do this before anything else. Any other suggestions?

Thanks,
Myc

 

What other work was previously performed to the braking system?

How many kilometres/miles are on the vehicle?

Assume you're in the UK. Did vehicle pass recent MoT? If so, were there any advisories on the braking system?

 

Hi,
I'm not sure about any history on the brake system, we've only just bought the car. It's got good history until 2016, then it's a bit sparse. It looks like filters and oil were changed last year.

Mileage is 115k

You are right, it's a UK car, and the MOT history is pretty good, think the handbrake was a problem about 6 years ago.
No brake issues in the last MOT.

 

Before replacing any parts, it's best to begin with a complete flush of the hydraulic lines and master cylinder. Replace the brake fluid with one that meets Jaguar specification and perform the tests listed above again.

Sometimes brake fluid that has absorbed lots of moisture can cause the brakes to have a poor pedal feel and travel.

 

Cheers, I thought the same, first job fluid flush. I'll report report back in a few days.