Brakes hardly working - xf disc all around

[XFChris 126]

Yesterday the brakes in my XF (disc on all 4 wheels) didn't work as well as they used to, the brake pedal was super spongy, and it took a lot of pedal travel to get the car to pull up. Previously the car would stop quite well.

I have given the car a quick look over, brake fluid in the reservoir was just about at the full mark, and there was no leaks I saw, aside from a weep at the back of the rear left caliper, where the handbrake plate bolts into the caliper body.

Aside from that weep possibly letting air into the lines, what else should I be looking at, as the cause of the poor braking?

[bear351c 10,273]

Sounds like an air bubble has worked it's way in. Spongy and inefficient.

 

Do you have a big vacuum booster on there.?? Is the vacuum line blocked, or fallen off ??

[XFChris 126]

It has the factory booster that is integrated into the master cylinder I believe.

 

I'd have thought if it lost vaccuum to the booster, the pedal would be very firm, but I will have a look at the rubber line during the week if I have time.

[user272 526]

I had a brake fluid leak on my old BA a few years ago, before It was found my mechanic said to test if its the booster/master cylinder to pump the pedal with the ignition off until it goes firm, then start the engine with your foot still firm on the pedal and see if it pulls in. Pulling in being normal operation.

[Valvebouncer 2,389]

There are two tests you do for brake boosters

1 with the engine off pump the brake pedal until it's gone firm, apply medium pressure to the brake pedal and start the engine. The pedal should sink a bit away from you if it's working correctly.

2 with the engine running press hard on the brake pedal and switch the engine off. The pedal should stay in the same position. If it's stuffed the pedal will push you back in the seat.

 

 

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[XFChris 126]

I just done the above tests , and the master cylinder checks out ok.

 

So that weeping caliper must have let in air over time :S

[Valvebouncer 2,389]

Sorry, I never read your original post.

You need to try and bleed the air out of the brakes. Grab a 10mm spanner and start at the furthest away from the master cylinder. Fill up the master cylinder, crack the nipple, make sure your comfy and watch. Don't pump the brakes, just watch. You will see the fluid slowly coming out and any air will come out as bubbles. This is called gravity bleeding. Even better, if you can get some clear hose that is a tight fit over the nipple, it will make it easier to see air bubbles.

Once done pump the pedal up, short pumps, don't do big long travel pumps as you can stuff the seals in the master cylinder going over crud. If it's back to normal you had air, if not your master cylinder has collapsed one of its systems - this is more likely than a system with no vacuum suddenly sucking in air while the car is stationary.

As for your weeping caliper, a more likely scenario is the flexible lines going porous, if so, replace all 3.

Good luck!

 

 

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[XFChris 126]

Thanks Valvebouncer. I gravity bled the brakes yesterday, and the brakes feel significantly better now, it's made the car driveable again. 

[Valvebouncer 2,389]

No worries bud!

It is a bit weird that it randomly got air in it, keep an eye on it.

 

 

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