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S-Type diesel, having just replaced rear calipers, we bled the brakes.

Now, with no engine, the brake pedal is as steady and firm as a rock, which agrees with the fact that the fluid coming from the bleed nipples was, as far as we could see, perfectly clean and with no air bubbles.

However, run the engine and the pedal can be pushed to the floor with only a modicum of pressure. One or two rapid pumps of the brake pedal restore the firmness of the pedal but for only a second until the same thing happens.

Can anybody help please?

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hi if you have replaced the brake pads, make sure you that you have wound the pistons back fully even though the pads might have fitted in partially pushed back, i seem to have read this somewhere about jag calipers, also remember to unscrew the resovouir as well because you are forcing the fluid back under pressure.

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could it be the master cylinder / servo diaphragm or perhaps a dodgy slave cylinder? any sign of brake fluid at each wheel, along the pipes?

not quite sure why it would change with the engine on?

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I've read many a tale about corroded pipes being an MOT failure, are you losing fluid? do you get any bubbles / movement of fluid in the tank when you press / pump the brake?

this is the bit i'm a bit vague on so bear with me, is the braking system pressurised by the engine? or is it self pressurising? I have an image in my head of a black thin rigid plastic pipe coming out of the master going somewhere towards the inlet manifold (from previous cars not the Jag) I might be talking rubbish but its bugging me !

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How to check a brake servo is working ok - With the engine off, press hard on the brake pedal. Turn the engine on and the pedal should depress slightly but no more than an inch or two.

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More info -

Know What a Brake Booster Does

The brake booster sits between the brake pedal and the master cylinder. When the driver pushes down on the brake pedal, the brake booster amplifies the force exerted on the master cylinder. The result is that the driver does not have to push down on the brake pedal very hard to engage the car's braking system.

A brake booster is a canister that consists of two chambers with a rod running through the center of it. This rod connects the brake pedal with the master cylinder. When your foot is off the brake pedal, the two chambers maintain a vacuum. When you push down on the brake, air enters one chamber while the other maintains a vacuum. As the air pressure in the one chamber increases, it exerts pressure on the diaphragm that separates the two chambers. This in turn exerts force on the rod that connects the pedal to the master cylinder. When you take your foot off the brake again, the interior of the brake booster returns to its original state, a vacuum.

Test if the Vacuum Hose is the Problem

Park the car. Turn off the engine. Pump the brakes five to six times using no more pressure than you would usually apply when braking. This action allows more air into the brake booster and bleeds any residual vacuum. Now, turn the engine back on, and keep your foot pushing down on the pedal. Note what happens to the pedal. With the engine running, the engine sucks air out of the brake booster, restoring the vacuum. Ideally, the pedal should fall towards the floor with you applying light and steady pressure. If the pedal pushes back against your foot, then there is a problem. This could be a blocked vacuum hose or a leak in the vacuum hose.

Test if the Brake Booster Can Maintain a Vacuum

The next test checks the integrity of the brake booster. If the booster cannot maintain a vacuum after you turn off the engine, the problem is the booster itself, and the whole mechanism needs replacing. To perform this test, park the car, take your foot off the brake pedal, and then start the engine. Let the engine idle for two minutes. Just before you turn off the engine, push down on the brake pedal and hold it. Turn the engine off, and then continue holding your foot on the pedal for another 30 seconds. If the pedal stays where it is, then the brake booster can maintain a vacuum. If the pedal starts to rise, it is time to replace the brake booster.

Examine the Vacuum Hose

It is the engine that creates a vacuum in the brake booster by sucking air out of the mechanism. If there is a vaccum leak in the vacuum hose, this could be the cause of the problem. There are various ways to test for a vaccum leak. Unlike a leak in a tire, you are not looking for evidence of air escaping, but rather evidence of sucking. Park the car, leave the engine to idle, and pop the hood. Take a can of brake cleaner, and spray along the length of the vacuum hose. If there is a vacuum leak, the running engine sucks in the cleaning fluid through a hole in the vacuum hose. In this case, just the hose needs replacing to rectify the problem.

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