My car went for its annual test and failed due to the front right (here, driver's) brake only braking about half as much as the left (293kgf vs 558kgf).

I'm working on it but would appreciate any ideas, too. I really don't want to have to replace the caliper! That was done just over 3 years ago (before I bought the car). Yes, same side - weird or what?

Here's a picture of a Brembo front caliper.

More pictures showing all the essential parts are here https://www.jaguarforums.com/forum/p...xkr-xk8-54781/
(they're not 100% identical but they have 4 pistons, the same pins and so on).

There are none of the, er, I think they're called "slide pins" or "guide pins" that I'm used to with other calipers. Also, no springs or locating lugs on the pads to fit them into the pistons.

I've cleaned them, checked all 4 pistons are moving (and moved them in and out several times), and will bleed them (they're due anyway), but any other ideas?

I can't quite (ha!) figure out a DIY way to test brake efficiency for each wheel. The car doesn't pull to either side, which surprises me but I suppose it's a big heavy beast.

 

So how are they measuring this? Are they measuring hydraulic pressure or are they measuring the actual clamp load of the caliper?

BTW, these Brembos aren't all that different than the racing versions I use on my track car just smaller and suitable for the road. Pulling them a part isn't a big deal.

 

Thanks, Bob.

They have powered rollers (with a rough surface) set into the floor, individually controlled. They spin the car wheel(s) and you apply the brake. The rollers measure the effect. (This is one of the mandatory annual tests for all cars here over 3 years old.)

I can strip them down but don't know what I'd be looking for. Brakes are not my strong point at that level

I understand there are no refurb kits (dust seals etc) or at least not apparently from Jaguar. But there's no sign of leakage etc.

Lots of material left on the pads and they seem equally worn. Actually, hardly worn at all. hmm, wonder if that's it - I try not to brake and do a lot of highway miles. Maybe now that I've worked the pistons some more they'll be OK.

 

When I got my Track car Maserati had installed a rear proportioning valve in the middle of an ABS operated system because they had done a cheapo by installing a huge set of Brembo racing calipers on the car but retained the stock booster, master cylinder and ABS unit with no system re-calibration. It called for a specific pressure setting out of that valve and when I got the car it was set to near zero. Nice huh?

So I ended up buying a somewhat inexpensive pressure measuring system for ABS system maintenance just so that I could calibrate that thing. I attached to the bleed screw openings (bleed screw is removed) on each rear caliper and with that you can check the actual pressures being delivered. It's designed for one man operation too.

But here's the thing. I'm no ABS systems expert and I think that the Jag probably has an Electronic Brake Force Distribution as well as ABS???

If the pressures are equal just sitting in the car with the engine running then it would seem to me that only the caliper is left as a cause.

Being able to run this by a true braking systems engineer from a major manufacturer would really help too. I did that with my race car and after one failed attempt at a system modification I learned a few surprising things.

The Brembos use a square cut O-ring for the seal. Once you know the size of the cylinder and groove dimensions in the piston I'm certain the fluid seals can be found from a Brembo supplier who knows what they're talking about. Same for the dust seals.

 

Thanks. I'll do some hunting!

 

The ABS brake power distribution motor may not be sending (proportioning) enough pressure to the front right channel. There is an ABS reset procedure. I believe this performed with the engine off, ignition key to the on position, I believe the brake pedal is depressed or whatever...the rest I'm not sure of. I saw a mechanic perform it on a 2000 Ford Windstar we've owned up to two years ago. jagv8, check into this.

 

Thanks. I should try AutoEnginuity in case it has an ABS reset I never looked for.

 

Don't quote me...dealer and/or independent repair shops don't want their customers to know about the ABS reset procedure mandated for all OBDII systems.

 

I doubt they're mandated - OBD II is only about emissions as far as governments (yours, mine, the EU, ...) are concerned.

Might exist, even so

 

Sorry, ABS def not OBDII emissions. Not entirely sure, but ABS and its controls/diagnostics may be mandated for all U.S. vehicles.

 

I cleaned it, worked the pistons, bled it, failed to find anything actually to explain it not working OK - but now it does

Thanks for the help!

 

Chalk it up to test station error. If it had really been that far off I think you would of noticed it.

 

"Blood Letting" (Bleeding) as a means to ridding the body of "Bad" humours during times of sickness, originated in Europe. Though not practiced today, mechanics have found it useful in purging air from hydraulic brake systems. Excellent news jagv8, and congrats on your moderatorship.

 

Bob - I'd not thought of that, though I'm not sure how it could be as I was present and saw the poor reading. Oh well.

Seth - thanks.

John

 

Good point Bob. As mentioned earlier, jagv8 hadn't noticed a biased pull to any one side.

 

I don't brake much, though. I really don't

 

LOL, you'll have life-time pads.

 

Brakes is for dose dhat don't knows hows to drive.

Maybe something was on your wheels or the roller or what ever ...

Anyway, you caught a break in this case ...

 

Maybe it was grease on the rollers or tires.

It could also have been a light glaze on pads or built up deposits on the rotors. A few hard rolling stops would have cleaned them up.

 

With my luck, diesel on the tyre!