Pulsing brakes

[willg54]

when braking today, more noticeable at slower speeds, it felt like the brakes were pulsing and felt as if they were coming on and off (not abs...)

I have 1 abs sensor playing up, could this be the cause? I'm going to give them another good bleed but with the MOT next friday and today my last day working on it before then, i'm at a bit screwed!

the porsche 993 brakes were immense though, stopped very quickly!

thought i'd clear this up a bit!

Symptoms
under braking, the car brakes in a straight line but jerks with hard and soft braking
has had a few abs errors with sensors but all currently working

Whats been done:
all brakes rebuilt - new seals etc
993 front brakes
re-bled several times - no air present
all pads are new
rear discs are new
front discs are skimmed (with in tolerance as far as i know, will check)

I've tried driving the car with ABS disabled and there was no change in the symptoms.

[driftin jo]

Most likely cause is you bomb is dead, the pulses you feel are the pulses from the pump, you don't feel that with a healthy bomb as you have a big reserve of pressure. The pump just tops this up.

[willg54]

Done the pump the pedal test and get over 40 pumps. Could it still be the bomb?

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[driftin jo]

I suppose what you are feeling could be the abs if it is coming on when it shouldn't due to a duff/ miss set sensor.

[Error404]

duff servo

[willg54]

Where is the servo? Any way of testing it easily?

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[Error404]

it is "behind" the brake MC. The factory test says if you take off the fluid return and more than a drop or two of fluid comes out then it has gone bad, but in my experience that isn't always a definitive answer. You can try driving it about a bit and also flushing the fluid can sometimes improve it but IME pulsing through the pedal is a bad servo. Not that bad a job to change if you need to anyway, just a bit fiddly. Good excuse to change to more reliable to vacuum assist

[willg54]

it is "behind" the brake MC. The factory test says if you take off the fluid return and more than a drop or two of fluid comes out then it has gone bad, but in my experience that isn't always a definitive answer. You can try driving it about a bit and also flushing the fluid can sometimes improve it but IME pulsing through the pedal is a bad servo. Not that bad a job to change if you need to anyway, just a bit fiddly. Good excuse to change to more reliable to vacuum assist

Are servos expensive?

I dont really have time to change to vacuum before next friday, only have this afternoon and friday morning before the mot

Going to rebleed this afternoon and sort the problematic abs sensor out.

[willg54]

yes...they're expensive!! MOT...cancelled!

[Error404]

yes new ones will cost a fortune, but you should be able to find a good used one pretty cheap if required. The fact that the brakes are working well is a good sign though, I would try a fluid flush and put a few miles on it before you do anything, might just be not properly pressurising or something, sounds to me like one of those little weird things you get when the car has been stood for ages

[willg54]

yes new ones will cost a fortune, but you should be able to find a good used one pretty cheap if required. The fact that the brakes are working well is a good sign though, I would try a fluid flush and put a few miles on it before you do anything, might just be not properly pressurising or something, sounds to me like one of those little weird things you get when the car has been stood for ages

lets hope so! only problem i have is that it's not road legal and i dont have the trailer for a while now so can't really test them might have to take a long drive to the MOT centre and see what happens.

The car stops very well, i was very impressed. just under slow speeds you can really feel the brakes grabbing as the pedal pulses.

are there any methods to help pressurise the servo?

[Error404]

Can you feel the pedal pulsing sitting at idle? If not I could indeed be ABS, you pull the fuse out of the ABS relay fuse under the dash to diable to ABS, that should help you to rule that out.

[willg54]

No, nothing at idle. I'll try pulling the fuse and see what happens then

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[chezz]

It's probably the sensor, dirty/faulty get a multimeter and check the resistance at the lead connection to the sensor(yellow) and you should get a poor reading on one of them...change/clean the offending lead.

[Donnie]

Could it be disk runout? Does the frequency increase with road speed? Try a DTI on the face of the disks.
Did you clean the hub faces properly when it was assembled?
2 Part disks (if applicable) can also generate runout if the bell is distorted or the fixing faces between rotor and bell are not perfectly clean.

If the pulsing is related to road speed I'd check for misalignment before forking out for new hydraulic parts at random.