Problem With Brake Pedal Travel

[kevd]

Hi All,
I seem to have a problem where my brake pedal reaches a solid mechanical stop on full depression, which is limiting my full braking potential. It is not really noticable in normal or semi spirited driving. But last week on a fast run down to Goodwood when approaching a corner at speed and braking hard I could feel that the pedal had come to a stop and I could apply no more braking force. Luckily enough braking to get round the corner but this is bit of a worry to me.
If I sit in the car with engine running I can depress the brake pedal and then you feel it come to a dead stop it is quite a mechanical feeling as though it is hitting a limit adjustment. When driving at normal speeds the brakes are good and I can just get to the point of locking the wheels as it touches the stop. The system has been bled numerous times, fliud changed, pads are all good and well bedded, braided hoses fitted all round. I've checked under the pedal and can see nothing that is restricting movement.
Any advice would be very welcome prior to pulling the pedal box out for investigation.
Many thanks
Kevin

[Panamax]

Are the callipers fitted correctly?

Does pumping the pedal help or not?

Otherwise everything seems to point to master cylinder and/or linkage.

[kevd]

The calipers are HiSpec 4 pots and appear to be fine.
No difference made if i pump the pedal, pretty much always the same.

[spitfire4v8]

When you say coming to a dead stop on the travel how much travel are we talking about exactly? there really shouldn't be more than say an inch of movement maximum. A really good well set up good condition system would have maybe half an inch of travel... enough to take up the mechanical clearances in the pedal pivots/servo, and push the pads that fraction of an inch each to put pressure on the discs.

[kevd]

Not 100% sure what the travel is, I will measure the pedal travel, probably have to be a weekend job now

[Skyedriver]

Reading the original post, it just that you are expecting too much of the braking system or is it a case of the fluid boiling?

You say in normal use it will lock up the wheels.....

[Hedgehopper]

Have you got excessive run-out on the wheel bearings that is creating more than normal pad knock-back?

[LLantrisant]

you changed to HiSpec without checking if your master is able to do the job? depending on the real fluid volume necessary to operate your hispecs the master may run out of travel and you may need a differtn master.

[blaze_away]

I have HiSpec 4 pots and the brakes work fine, also fyi just replaced master and servo.

I would say you are reaching the mechanical end of the piston travel in the master cylinder.

Bleed all the system, gets fresh fluid everywhere then see how it is. Probably need a new master or new seal kit fitted.

[Classic Chim]

Try bleeding the master cyl
The nipple that’s really hard to get too.
If that doesn’t work suspect a failing master cyl/ servo as there is nothing else in the system other than pipes and your Calipers/pads that can go wrong.
With my Brembo upgraded brakes my pedal does travel slightly longer than standard, say 7mm but the brakes lock up long before I’m anywhere near the stopper or the end of pedal travel.
I suspect air in your master or it’s failing.

[s p a c e m a n]

Whatever is on the inside of my servo broke and I had exactly the same thing which ended with me driving up a kerb to miss a line of stationary traffic. I could only get half of the normal travel with the pedal, but as I didn't have another chimp to compare with I thought that it was all in my head, so we tried a few things until I bit the bullet and pulled the pedal box out.

The servo looked fine externally, no rust and the master cylinder hadn't been leaking. After smacking it on the bench and shaking it about a bit the pin finally depressed all of the way like it was supposed to.

I bought a new one from racetech.

It was like having constantly cooked pads, the pedal felt fine and I had the braking capacity that I needed when driving, it was only when trying to come to a stop that it was obvious that something was wrong.

It was when trying to bleed the brakes that we finally realised what was going on, we couldn't get a full press of the pedal and the set up is so basic that it was easy to see that either the servo or master cylinder was getting stuck.

Edited by s p a c e m a n on Thursday 11th November 08:34

[kevd]

Thanks for all the replies, some great advice. I am pretty confident that the system is bled thoroughly including the one on the master cylinder. My symptoms sound pretty much the same as Spaceman describes, it is looking like i'm going to have to bite the bullet and get the pedal box out for closer examination, which looks like a bugger of a job. I will let you know what I find.
Thanks Kev

[spitfire4v8]

Before you start taking the pedal box out, which is a swine of a job at the best of times, why not let someone else drive the car and give their verdict .. it might, or might not, be the master cyl / servo .. and if it isn't then you've just wasted a whole load of time and money.

Good to see you had a mk1 spit though! I've had mine for over 40 years.

[Zener]

What Steve said above also you could try loosening the brk master cylinder from the servo body two 17mm nuts just 1/4" to confirm its the servo push-rod internals binding/jamming or a failing/faulty master cylinder if the pedal travels further without the locking pedal dont drive car like this however