Limited brake pedal travel

[cinquecento]

Had a slight moment braking today..

When applying a fair bit of pressure to the pedal today, I noticed that my Griff was no longer slowing down proportional to the amount off additional pressure I was applying on the pedal. I then REALLY stood on the breaks only to find the car slowing and the same rate. Luckily no damage done.. but on a quieter stretch of road I did some brake tests.. and found there is a certain point which occurs after approx 2 inches of pedal travel, at which the brake pedal can not be depressed any further. It’s like the pedal has hit its stop. Press the pedal any harder and it doesn't travel further.. and the car slows at the same rate.

So, my question is with the master cylinder, are there any known issues where the cylinder piston jams or has its travel restricted/limited. Could this be anything else.. as the idea of pulling my pedal box and master cylinder out if not that pleasing


Many thanks

[Gsw500]

Have you checked the brake fluid level..?

I had same symptons last year and nearly stuffed it into the back of a fezza 348..

We found that the fluid had gone hydroscopic and after a full bleed and I mean a complete replacement of fluid, car will now stop on a sixpence..


Gsw500

[5.0ltr]

Is this like when beng towed without engine power, really hard pedal and lots of effort to stop? In which case may well be servo.

[HRG]

What happens to the pedal when you test it with the engine off in the garage? I'd get down and look for any obstructions in the footwell first.

[GreenV8S]

A scored master cylinder could cause that sort of problem, or it could be a servo failure.

[cinquecento]

Thanks All,

Brake fluid is fine (res is full) and when I the apply the brake then start the car, there is a softening on the pedal.

May I also ask how the master cylinder divides the 4 brake lines. Are the front and rear brakes operated in pairs (the 2 fronts together and the 2 rears together) .. or is it one front and the diagnally opposite rear are operated in pairs.
I also assume the two connected pistons in the master cylinder mentioned on p 160 of the bible would drive one pair of wheels.. piston A drivers fonts and piston B drives rears.

I too am starting to think I may have a seized piston.

[GreenV8S]

cinquecento said:

May I also ask how the master cylinder divides the 4 brake lines. Are the front and rear brakes operated in pairs (the 2 fronts together and the 2 rears together) .. or is it one front and the diagnally opposite rear are operated in pairs.

It's a tandem master cylinder with two chambers separated by a floating piston. One chamber drives the front brakes, the other drives the rears (via the pressure limiting valve). As I understand it, the point of the floating piston is that if one circuit starts leaking it will limit the amount of fluid that can be lost to that circuit, allowing the other circuit to carry on working.

All the usual failures would result in the pedal going soft rather than locking up. Given that the servo is working, the most likely explanation is that the master cylinder is scored and the piston is catching on the scored surface and jamming in the bore.

[HRG]

GreenV8S said:

cinquecento said:

May I also ask how the master cylinder divides the 4 brake lines. Are the front and rear brakes operated in pairs (the 2 fronts together and the 2 rears together) .. or is it one front and the diagnally opposite rear are operated in pairs.

It's a tandem master cylinder with two chambers separated by a floating piston. One chamber drives the front brakes, the other drives the rears (via the pressure limiting valve). As I understand it, the point of the floating piston is that if one circuit starts leaking it will limit the amount of fluid that can be lost to that circuit, allowing the other circuit to carry on working.

All the usual failures would result in the pedal going soft rather than locking up. Given that the servo is working, the most likely explanation is that the master cylinder is scored and the piston is catching on the scored surface and jamming in the bore.

Or that there is a physical obstruction to pedal travel.

[GreenV8S]

HRG said:

GreenV8S said:

cinquecento said:

May I also ask how the master cylinder divides the 4 brake lines. Are the front and rear brakes operated in pairs (the 2 fronts together and the 2 rears together) .. or is it one front and the diagnally opposite rear are operated in pairs.

It's a tandem master cylinder with two chambers separated by a floating piston. One chamber drives the front brakes, the other drives the rears (via the pressure limiting valve). As I understand it, the point of the floating piston is that if one circuit starts leaking it will limit the amount of fluid that can be lost to that circuit, allowing the other circuit to carry on working.

All the usual failures would result in the pedal going soft rather than locking up. Given that the servo is working, the most likely explanation is that the master cylinder is scored and the piston is catching on the scored surface and jamming in the bore.

Or that there is a physical obstruction to pedal travel.

Hard to imagine what obstruction there could be, but I agree it's worth having a look at the pedal box just in case.

[HRG]

GreenV8S said:

HRG said:

GreenV8S said:

cinquecento said:

May I also ask how the master cylinder divides the 4 brake lines. Are the front and rear brakes operated in pairs (the 2 fronts together and the 2 rears together) .. or is it one front and the diagnally opposite rear are operated in pairs.

It's a tandem master cylinder with two chambers separated by a floating piston. One chamber drives the front brakes, the other drives the rears (via the pressure limiting valve). As I understand it, the point of the floating piston is that if one circuit starts leaking it will limit the amount of fluid that can be lost to that circuit, allowing the other circuit to carry on working.

All the usual failures would result in the pedal going soft rather than locking up. Given that the servo is working, the most likely explanation is that the master cylinder is scored and the piston is catching on the scored surface and jamming in the bore.

Or that there is a physical obstruction to pedal travel.

Hard to imagine what obstruction there could be, but I agree it's worth having a look at the pedal box just in case.

You don't have young kids, do you

Over the years I've learned that anything can end up anywhere... Admittedly it's far less likely in the Griff, but it only takes a few seconds to eliminate a physical obstruction.

[Yellabelly]

Hard pedal is usually a sign of servo problems, the whole point of the servo is that it multiplies the braking effort that you would need to apply if it was non servo, hence the hard pedal. The softening of the pedal on start up is usual, as the servo starts to build the pressure, to operate the brake, it provides the assistance to braking effort which you feel as a softening pedal.

IMVHO if this is something that has only just occurred, it is more likely to be servo related than scored cylinder (sorry Peter).
Have you checked the vacuum pipe to the servo to see if it is deforming under pressure and therefore not giving you the vacuum?

YB

From the VX220.org site "a servo fault would give you a loss in braking power combined with a hard pedal."

From the miniforum "With the car off, press the pedal several times, the pedal should be very hard to press after 4 or so goes. Compare this to how hard the pedal is to press with the engine running, if its even close to the same then your servo is buggered and not assisting the brake......Otherwise check the vac lines incase its split or replace the servo unit"

Edited by Yellabelly on Thursday 12th June 18:47