Partial brake failure

[taylormj4]

Have this odd problem with the brakes. A TVR independent has checked them and found nothing but today I had partial brake failure and couldn't hold the car at stop on a hill.

The brakes appear to work normally when driving, i.e. brake pedal movement and travel appears normal but if the brake pedal is held on, it sometimes very slowly drops. Today I was holding the car on the brakes in traffic on a steep hill and if the pedal was held in the same position, the brakes would gradually come off and the car roll backwards. Push the pedal more and it would hold for a few seconds then roll again.

Experimented at the bottom of a hill and found that the pedal does eventually go hard and no more travel but pedal is a lot lower than normal. It the brakes are released and reapplied, the pedal stop comes back up and then the slow drop begins again.

If I let the pedal drop to the lowest point when it eventually goes hard, the brakes will stay on and no further drop but if I release them a bit then try to stop the car, a lot of effort is needed.

Is this a servo issue? I thought servo just reduced the effort needed to brake not the pedal travel point at which the brakes come on and the pedal goes hard?

Thanks.

[phazed]

Sounds like the servo, pretty defo imho.

Check that you have full vacuum, possibly a split vacuum pipe. ?

[Belle427]

I would usually associate a sinking pedal with fluid leak or a worn master cylinder.

[Steve_D]

Belle427 said:

I would usually associate a sinking pedal with fluid leak or a worn master cylinder.

If checks have been made and there is no leak then this will be master cylinder seal failure. In this case the fluid is pushing past the seals but staying inside the cylinder. If this is a combination seal and cylinder wear then that will be why they hold OK when lower down in the travel.
I never replace seals just fit a new master it's not worth taking chances.
Reference the leaks...under the car or at the wheels are easy to spot but a leak from the master will run down and hide in the carpets so you need to check there by pulling the carpets out.

Steve

[spitfire4v8]

With a tandem master cylinder (dual circuit / split circuit systems) it's very possible to have one seal leak for one circuit and the pedal sink slowly to half way but then the other seal being ok which holds pressure correctly to the other circuit.
If the good circuit was the pressure-limited rears you'd not get very good brakes even with yor foot hard on the pedal, albeit your pedal is now at half way down its travel.
You say you had the brakes checked out annd they didn't feel this effect? This is like first week of college stuff..

Edited by spitfire4v8 on Tuesday 12th June 09:14

[magpies]

Agree with both steveD and spitfire4v8

This happened to me lot of years ago when I was young and stupid - with my rally mk1 RS2000 on an event. just before half way halt the car would not gently brake as peddle would sink to floor but would work if hit hard - actually won the event

Resulting strip down showed twisted/deformed seal in the master cylinder.

[Sardonicus]

Yep text book MC internal leak as mentioned already witnessed more times than I care to remember considering the age of some of these cars they are still wearing original fit equipment like this but there is only so much seal conditioning/saving regular brake-fluid changes can do

[taylormj4]

spitfire4v8 said:

With a tandem master cylinder (dual circuit / split circuit systems) it's very possible to have one seal leak for one circuit and the pedal sink slowly to half way but then the other seal being ok which holds pressure correctly to the other circuit.
If the good circuit was the pressure-limited rears you'd not get very good brakes even with yor foot hard on the pedal, albeit your pedal is now at half way down its travel.
You say you had the brakes checked out annd they didn't feel this effect? This is like first week of college stuff..

Edited by spitfire4v8 on Tuesday 12th June 09:14

Yes dissapointing. I actually suspect they forgot to check but I hoped that they would have owned up to that rather than telling me they were all ok, especially as it was brakes.

[taylormj4]

Other thing to add is that there have been two occasions where I think fluid has run out of the master cylinder.

This is evidenced by paint damage on the wing below the master cylinder. The fluid level looks correct in the master cylinder.
If anything there was too much as when I put the cap back on, I was a bit enthusiastic and the sensor under the cap pushed some fluid out, which overflowed down the wing.