Grande Punto scary brake issue. Help?!

Hi everyone.

We bought our 2008 Grande Punto 1.9 M-Jet Sporting a few months ago when we probably should have walked away (see key post, etc...). However, we are where we are, and the latest - pretty troubling - issue concerns the brakes.

When you brake (at any speed - it's happened to me at around 50mph and to my wife at under 10), the nearside rear wheel locks with very light pressure on the pedal and before the ABS cuts in. This has the effect of sending the car sliding across to the left, which is quite unnerving. Not to say potentially lethal.

No warning lights are coming up on the dash and no codes on the diagnostics.

With the help of our local garage, we have so far tried changing all discs and pads (they needed it anyway), purging / bleeding the circuit, swapping tyres front to rear and side to side to see if wear was a factor. It's not, still always the nsr wheel locking.
And the latest thing we've tried is a new ABS pump to see if it was sending the wrong pressures to the different wheels. No joy.

We are reaching the end of our patience; the car was bought to be a courtesy car for our business and until this is definitively sorted, we can't allow customers to drive it.

Our next step is to change the nsr caliper to see if that helps, but it's a slim hope.

Does anyone have any bright ideas as to what might be causing the issue?

I feared a bent/cracked chassis, but apparently that's unlikely.

All constructive thoughts welcome!

I presume that it'll have a mechanical proporting valve on the back; Are those free? I seem to recall that the Puntos have two. One left and one right. I could be wrong.

Can the diagnostics you've got run the pump on? Make sure you get any air out.

If that doesn't work have a look for a bent stub axle. That can cause the disc to snatch when you brake. Obviously you don't need too much force to lock a rear wheel on a hatch. You should be able to get a feel for how the rear brakes are acting via the handbrake. ie if one side is over assertive then you could have a mechanical problem

Thanks szkom, I'll pass on your thoughts.

Although you don't need a lot of braking force to lock up the rear wheels on a hatch, you generally need *a lot* more than this currently takes - it locks considerable before the ABS cuts in. My scariest moment was when slowing moderately from around 50mph before a right hand bend to avoid an oncoming bus that was over in my lane. Luckily, the car slid to the left, or I'd have been history.

I guess the point I was making about the force needed to lock the rear wheel is that as it is a relatively small force compared to the front it is quite easy to overlook the obvious and assume that something is correct. If this was the front axle we'd be seeing glowing brakes, bulging hoses and any number of visual clues.

Have you had it on a brake tester? Give you an idea what each wheel is pulling. Be interesting to see if you have an imbalance on the front also.

It could well be a problem with the caliper itself, as I know of a similar experience with a Grande Punto, albeit the wheel didn't lock-up instantly, but only while braking more aggressively. As @szkom said, you may check how the rear brakes feel via the handbrake, but it's not definitive.

Regarding the mechanical valves, I don't think that the Grande Punto had one, since it has an electronic brakeforce distribution. The lines should go directly from ABS to the wheels, without any valves in between.

If you have an OBD reader, there's a software called MultiECUScan, and you can hook it and see the wheel speed sensors, as well as what the ABS control unit is doing. But, be aware that when you go into the ABS control unit, it will temporarily disable the EBD, so be VERY LIGHT on the brakes, as they will feel even more agressive.

TL;DR, check the caliper first, as you might be surprised to see that the problem is actually there.

And oh, check for correct tension of handbrake cable on both calipers also!

Also check the condition of the rear brake hoses, sometimes they can collapse internally and give problems (eg: if a hose has been clamped in the past...)

Not sure it's related, but the handbrake is dreadful...

Which is surprising if you've had new discs and pads.

It can be related, as the handbrake tensions the same pistons as the hydraulic brakes from the pedal. In my case, the handbrake was tensioned up on one caliper and it caused uneven brake engagement.

Eh? Aren't we forgetting that the design is similar to the Coupe, with a mechanical handbrake acting on a lever that pushes in a piston, the same piston that's likely stuck in the "just about to lock up" position? Changing discs and pads is no guarantee to "fix" weak rear brakes/handbrake...

Similar, but different. Discs are circa 20mm bigger as a starter. And you couldn't have fitted the new pads if the piston was that far advanced. A decent mechanic should've seen a stuck piston/sliders when doing the work.

I should have specified that it was the front discs and pads that were changed, not the rears.

Definitely I'd get them sorted next. Dave probably has hit it on the head; sticky piston.

Or perhaps a little more likely, sticky calliper guide pins?