Brake pedal effort - help please

[nawarne]

Help please.
The amount of effort required to operate the brakes on my S seems to vary. Sometimes, I'm almost standing on the pedal - yet at other times/mostly - normal pedal effort is effective!
I guess I should be looking at the master cylinder and/or the servo - The calipers are all OK last time I looked. Any pointers welcome!
Nick

[ceejay73]

hmmmm interesting... Would suggest first checking vacuum pipe from intake manifold to servo for any splits, perishing or blockages as any of these will affect the assistance from the servo. Also look to see if it is collapsing at all, this would probably be most obvious when the throttles close directly after a good rev as this is when there will be the most vac pressure. If the pipe looks dubious in any way would advise changing it, probably the cheapest place to start anyway. Oh and listen around the servo for any hissing noises as if you have a split diaphram you may well be able to hear it leaking. Hope thats of some help.
Good luck,
Carl.

[honourablepickle]

try starting the car with your foot on the pedal - if the servo is working you should feel the pedal go down a bit - although it sounds like the mastercyl is leaking a bit past the seals though .........
reb

[nawarne]

Thanks guys,
Seems that I'm looking in roughly the right area. It does seem that when it occurs, there is zero servo assistance i.e. the pedal requires loads of effort to work and the pedal is 'hard'.
Nick

[nawarne]

OK - an Update.
- I've checked the vacuum hose and appears to be fine. Non return/1 way valve also checked and OK

- Did the check with foot on brake pedal and start engine - pedal sinks about 1/2"....so servo ok?

- Reb - surely if master cylinder was nannied, pedal would be really soft as fluid 'blew' around piston/seals?

- Spoke to guys at tvrcarparts. Their advice was to remove back wheels and check operation of rear calipers, if not a lot happening, suspect pressure limiting valve/device. Or get car onto brake test rollers at MOT test station and compare brake effort front to rear. This I did and not a lot of difference. In fact, the 'tester' said that the brakes were ok but the rears were not putting up quite the same as the front. The guy also thought that the 'operation' improved as the pedal was 'worked', and he suggested lubricating the pedal box fulcrum.

So my thoughts are to look at the rear calipers....and then suspect the rear bias valve??? Any other suggestions.
Nick

[GreenV8S]

The rear bias valve appears to be of the inertial type, so it's entirely possible that it behaves differently when the car is moving.

[honourablepickle]

this is fun trying to guess problems over the internet !
if the pedal goes down as you start the car then there is servo action so servo probably ok - they either work or don't usually
the reason i thought the mastercyl could be a problem is that failing seals don't always leak past and make the brakes soft - sometimes bits clog up the outlets which efectively reduces the amount of fluid movement and makes the pedal hard - try bleeding all corners and making sure there is a free flow of fluid - this might help highlight a problem with the bias valve if it is really knackered
the idea of going to a mot bay and using the rollers is a good one and will at least show up sticking calipers etc
have fun
reb

[nawarne]

Richard,
OK, thanks, hadn't thought about muck in the master cylinder.
Think I'll have to have her up on axle stands on Saturday then and bleed at all 4 corners/check piston movement in the calipers.

Bottom line is that I CAN stop .... but I'm having to really push on the pedal. The MOT chap said the brakes ARE ok - its the feel I have an issue with.
Nick

[nawarne]

Further update (2)!!!
OK, put the car up on stands on Saturday. Both back wheels off. Double checked calipers,- with pads removed & calipers clear of disc/rotor, pistons moved out fine. Did notice offside piston a 'bit' harder to screw back in (when re-assembling), compared to nearside. Repeated process to ensure piston not seized.

Bled both sides (rear). Got a bit of crap in with the 'spent' fluid from both calipers. Then bled both the front calipers - did not check movement of piston(s) as didn't have 7mm allen key to remove slide pins and lift caliper off disc.

Did notice all discs are shiny - indicating pad contact/operation.
Re-tried starting engine with foot on brake pedal - again noticed slight sinkage/movement when engine fired-up. BUT also noticed pedal rising with quite some force when engine turned off. I checked this out with my local garage and they say this IS an indication of a servo fault......Which takes me back full circle!!.

On the upside I noted that the chassis treatment and waxoyl is still holding up well. Couple of patches that will get some treating next w/end - but otherwise sound. On a downer I noticed that the o/side front damper is just starting to weep ---it's not yet 2 years old (AVO) looks like I'll be speaking to 'gazman' Derek!
Nick.

[honourablepickle]

hmm......strange symptom that.
is seems to suggest that air is leaking in somewhere
try putting a vacuum pump on the servo hose and see if it loses preasure ?
i changed my servo last month and had problems getting a new one - finally fitted a chimera one which is similar apart from the clevis
let us know the result of a vac test
regards
reb

[nawarne]

Result!!!!
Changed the servo today and all systems go.
The old one was absolutely rotten on the underside - now normal service is resumed!
Thanks for all the support meanwhile
Nick.