An odd braking problem (car)

If, when you brake in a car with discs front and rear, the car slows, but feels like its not doing it evenly, whats the cause?

By "evenly", I mean that say you're doing 20mph, then apply firm even pressure to the pedal, it feels as though you're pumping the pedal slowly, so its slowly throwing you forward gently as you come to a stop, but slower and slower as the car comes to a stop.

Hard to describe.

Its something to do with the revolution of the wheels (obviously) because, as above, the slower you get, the less frequent the "pumping" / "lunging" forward sensation.

I'm guessing its an issue with the front brakes, rather than the back, due to the momentum I wouldnt feel it as much if it were the rear.....?

[the_lone_wolf]

if it were a bike i'd say check that the disc isn't warped, but i don't think you'd easily be able to bend a car brake disc

edit: unless they overheat, haven't been descending long hills towing an unbraked trailer in 5th lately have you?

What car is it?

I know ours has an "exhaust assisted braking" which means the pedal has some strange effects.

For example, hold the pedal lightly, and it'll slowly sink to the floor without any additional pressure. But stand firmly on the brake & it barely moves. Brake force can be adjusted by pumping the pedal.

Your's sounds very similar to ours.

Originally Posted by the_lone_wolf

i
edit: unless they overheat, haven't been descending long hills towing an unbraked trailer in 5th lately have you?

No, I only use neutral for braking.



Is it just a case of removing the wheels and checking for bentness, or is that kind of thing not obvious to the naked eye at 90 degrees?

if you can feel pulsing it generally means that a disk is warped.

Originally Posted by ooger

Is it just a case of removing the wheels and checking for bentness, or is that kind of thing not obvious to the naked eye at 90 degrees?

Yup, and for a trained eye, it's easy to spot. For an agerage joe, it's not so easy (unless it's extreme warping).

Originally Posted by Baph

What car is it?

Its a Volvo...? I've never felt anything like it (apart from when you're in with a 17 year old, then you expect it)

[the_lone_wolf]

Originally Posted by ooger

Its a Volvo...

so it's like towing an oil tanker all the time then?




sorry, can't help more

@Baph - mine does the sinking pedal feeling, very unnerving if you're allowing the car to roll slowly in traffic on a slope and you suddenly realise the pedal is at half travel and you haven't stopped

Originally Posted by ooger

Its a Volvo...?

Some Volvo's (esp newer ones) are fitted with VCB (Volvo Compression Brake) and linked to VEB (Volvo Exhaust Brake). This means that braking is relative to the RPM of the engine at the time. It's very similar to the stuff that's fitted to my Ford Galaxy.

Simple checks really...

At a standstill, with the engine idling, put your foot on the brake moderately. Hold that pressure constant for 30secs... does your foot creep to the floor? If it doesn't, it's not VCB/VEB. If it does, press firmly on the brake & the revs should rise a little, and resistance will become pretty strong in the pedal.

If the above paragrah holds true to your car, it's a case of "They all do that sir."

Originally Posted by the_lone_wolf

@Baph - mine does the sinking pedal feeling, very unnerving if you're allowing the car to roll slowly in traffic on a slope and you suddenly realise the pedal is at half travel and you haven't stopped

When you're at half travel it's no big deal. If you need to stamp hard on the pedal, the revs will rise to apply more braking force. Then if you need to lift off a little & push the pedal again.

T'is the advice that the dealership gave me when I queried it.