An odd braking problem (car)

[ooger]

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.....?

:smt102

[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:eek:

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

[Baph]

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.

[ooger]

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

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?

[custard]

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

[Baph]

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).

[ooger]

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]

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

[Baph]

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."

[Baph]

@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.

[caz650s]

Sounds like a warped disc to me ...more likely to be a front one

[SV-net]

Yep Im in the gang for the warped disc.
You must remember when replacing them to do them as pairs and make sure that the hub face is clean of any debris as this can affect the new disc.

Discs are not expensive, more a pain to get under there and do the job really.

[Bibio]

could also be a cracked ABS ring, but this will show up as a fast pulsing under your foot.

could be worn or badly seated pads.

could be a siezed hand break cable (caused a disc to warp) if the cable is seized and the disc is warped i would sujest you get the caliper checked aswell, over cooking the caliper for a length of time will bugger the seals.

other tell tail signes of a siezed h/b cable or warped discs are blueing and or heavily scored discs.

easy way to check a warped disk (well for me) is after a run you will find that 1 wheel feels a lot hotter than the rest.

another is that your car will feel as if it needs your wheels balanced when you are at any speed.

either way get it checked by a professional. whats your life worth?

[ooger]

thanks, will check and update you all later.

Today, I ride.

[Baph]

easy way to check a warped disk (well for me) is after a run you will find that 1 wheel feels a lot hotter than the rest.


Yup, just be careful doing that though. One of the lads came into work yesterday after touching his disc. He was sat with his finger stuck in a cup of water all shift. :lol:

[SV-net]

He touched his brake disc after using the vehicle Hmmm T**t springs to mind!

[rowdy]

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:eek:

edit: unless they overheat, haven't been descending long hills towing an unbraked trailer in 5th lately have you?:confused:
It is possible to warp car discs easy enough especially if they have been replaced with low quality pattern parts, cars run a dual circuit that is divided diagonally i.e.front nearside rear offside, front offside rear nearside so that if the system malfunctions it shouldn't just throw you off the road. I've had a couple of volvo's does it have abs brakes? these are also fitted with ebd(electronic brakeforce distribution) so the possibility is this is playing up although warped discs do give the peddle a pumping sensation do they vibrate when braking from high speed?