Odd Brake Behaviour

[Bailes1992]

So my Father has been asking me for a few weeks to take my Mothers car out for a drive because the brakes seemed a little off.

The car in question is a 2021 MG3 with 12,500miles on the clock. My Mother is a District Nurse so lots of short stop/start journeys that are quite hard on the car. She's not the most sympathetic of drivers either.

I took the car out for a drive and at slow speeds (up to 30/40mph or so) the brakes seem absolutely fine and perfectly adequate. However, I couldn't get the ABS to kick in at all when braking unless you absolutely lunged at the brake pedal at a very slow speed (sub 20mph or so).

When you start picking up speed the brakes get a bit weird. They brake to a certain point, but then you find no matter how much harder you press the pedal, the stopping power doesn't increase.

I took it out on a quiet and empty dual carriageway, accelerated up to 70mph and anchored on the brakes down to 20mph repeatedly for a few miles. That seems to have brought the brakes back to life quite a bit. By the time I got the car back to my parents house I could easily get the ABS to kick in at any speed.

I'm just wondering what happened? My only thought is that the brake pads had 'glazed' and couldn't create the necessary friction. Maybe I managed to clean this up a little bit with some heavy braking.

 

[Jetronic]

EBD happened. It's sensing wheels are about to lock up, and is preventing more pressure to reach the affected wheels (which can be all of them).

You can push through it, but you have to be hamfisted.

Of course, to give EBD a chance to do this, the brake force and tyre slip has to increase gradually. Glazing, or any other reason for lower friction at the pad/disc interface would help EBD to control the pressures.

Very likely EBD is toned down or inactive at very slow speeds, as you would have no braking on ice or snow otherwise.

 

[Bailes1992]

EBD happened. It's sensing wheels are about to lock up, and is preventing more pressure to reach the affected wheels (which can be all of them).

You can push through it, but you have to be hamfisted.

Of course, to give EBD a chance to do this, the brake force and tyre slip has to increase gradually. Glazing, or any other reason for lower friction at the pad/disc interface would help EBD to control the pressures.

Very likely EBD is toned down or inactive at very slow speeds, as you would have no braking on ice or snow otherwise.

I may have been inclined to agree with you had I not experienced the brakes improving over a period of time that I drove it.

 

[Jetronic]

I'm sure they did improve. Let's assume the brakes are a bit off...

EBD allows a certain % of tyre slip before intevening. Let's call it 10%, with 0% being a free rolling tyre and 100% a locked up tyre. If one wheel is showing near 10% slip the hydraulic line is closed, or if it's more pressure could even be reduced to bring it back to 10%. No matter what you do, that wheel won't brake any harder as long as the system is active.- Gradually the other wheels will get to 10% aswell and they also get capped. Until you try to push through it (emergency brake assist kicks in) but it won't be likely until all 4 are showing high slip values.

If 2 wheels are underperforming (say at the rear with the weaker brakes) it will be very hard to get through it on the road. If you removed oxides or glazing or unstuck pads or calipers over time, it would be much easier, and the brakes would perform with less pressure too

 

[PontiacHO]

ABS module had seized solenoid/valve. Hard braking freed it up.

 

[Pew]

I'm just wondering what happened? My only thought is that the brake pads had 'glazed' and couldn't create the necessary friction. Maybe I managed to clean this up a little bit with some heavy braking.

ABS module had seized solenoid/valve. Hard braking freed it up.

I would agree with this. I doubt the pads were glazed during normal daily driving at your guy's speed limits unless multiple hard stops from high speeds were done in succession if nothing else in the system is broken. I'm assuming it's still on the factory pads and rotors with that low amount of miles?

 

[oggb]

The Hyundai in my signature used to exhibit the exact same braking behavior when it was driven almost entirely in the city for several years. The only way I could fix it was by essentially bedding in the pads as though they were new every few months.

It didn't help that my wife drove the car 95% of the time and she brakes for stop signs/red lights as though she's braking for an apex after a long straight on a racetrack. Every stop for her feels like a panic stop.