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Diagnose My Brake Problem

iplen
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iplen Mentor

iplen

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Took my 32 to Wakefield a few months ago, and had a different brake issue. The car is v-spec ii, and was running brand new TRW GP 600 fluid with PMU club racer front pads (don't know what the rears are..). I would get 8-9 laps into a run, with no probs at all, solid pedal, stopping like nothing I have felt before then the next corner the pedal would go to the floor.. It would come back the second or third time, and it would stop hard when the pedal came back.. Knockoff seemed to be the obvious answer, but I would do 2-3 laps slowly to let everything cool, go to the pits, where sitting still it would do the same thing. 30 seconds without touching the pedal, straight to the floor, second or third press I would have a full pedal.. This happened 3-4 times, any ideas??

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Hanaldo Grand Master

Hanaldo

Posted
Posted

My money is on your master cylinder. Possibly brake booster as well, but I'll commit and say master cylinder. I'd suggest with the car off and parked, pump the brakes a few times and then hold them on firm. Does the pedal change feel or move at all?

Duncan Grand Master

Duncan

Posted
Posted

You sure you are not boiling your brake fluid?

Was ALL the fluid changed? Might be worth a long bleed and top up with the high temp fluid. There may be old fluid in the system still.

That's my guess on most likely issue, particularly if you get a pedal back after a pump or 2. If it has boiled at some stage and left air bubbles, the first pump or 2 brings the pressure up then you have a reasonable pedal.

Unlikely to be pad knockoff in a skyline, they have good wheel bearings and don't tend to move much.

Sounds like it BUT its got the 600 stuff so it would be pretty hard to do? :/

True....it would be harder, but you can do anything if you try hard enough. I have boiled every 600+ brake fluid at different times over the years. Some last a little longer but it's not really that hard to get 600+ degrees in brakes. For what it's worth, I've found Agip to be the best, but these days I use motul because it is almost as good and cheaper (and I don't need that last 1/10th any more as I now have a better setup since some rule changes)

iplen Mentor

iplen

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With boiling fluid I would have expected to feel some fade prior to the pedal going completely, have got all the old stuff out (used a whole litre of fluid on the day) we were bleeding it to get the pedal back as well, we are tying the PMU fluid next time, but will try the master cylinder test when it is running again. Brakes were rediculously hot..

djr81 Grand Master

djr81

Posted
Posted (edited)

With boiling fluid I would have expected to feel some fade prior to the pedal going completely, have got all the old stuff out (used a whole litre of fluid on the day) we were bleeding it to get the pedal back as well, we are tying the PMU fluid next time, but will try the master cylinder test when it is running again. Brakes were rediculously hot..

Why would you expect to have fade prior to boiling the fluid? Fade is a pad issue not a fluid issue. The pad can be perfectly happy whilst the fluid is boiling its nuts off.

Bleed them again - you may well find the stuff comes out black. Usually does for me anyway.

Oh and get some ducted air onto the rotor/calliper etc.

Edited by djr81
iplen Mentor

iplen

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Wrong wording, would have expected the pedal to feel a bit soft before going straight to the floor. All ideas so far were ones we had considered but none seem to line up 100%. Was hoping someone might come up with something left field. We will put some ducting, check out the master cylinder and run some better fluid next time

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