Land Rover Repair: Master clyinder failure??, model discovery, salvage yard

Question
QUESTION: On an extremely hot day my brakes failed petal went to the floor would not pump up.checked for leaks no blown lines or leaks reservoir full. Changed out master cylinder replaced fluid bleed brakes all ok, now 2 months later a repeat, leading up to this was 30 min stuck in traffic AC running, with petal completely to the floor some slowing occurred, pumping helped, held pressure to stop.
    What could be the cause, I'm thinking weak seals in the master cylinder along with the heat, (grabbing the master cylinder it was hot).   what are your thoughts?
Thank you
Charles T

ANSWER: On a later model Discovery I would look at the ABS / traction control modulator assembly.  I'd guess one of the valves is giving way and creating the symptom you describe.  The idea that you had a failed master cylinder and swapped it for another which failed the very same way is not plausible.  That leaves the modulator as the likely and possibly culprit.

If the vehicle was here at Robison Service we would try reading the valve positions on the test system; you might end up having to swap it for a sure diagnosis.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your reply, Question what is referred to as a late model discovery, This one is a 95 model year,  Is there a way I can test out the modulator? Are they rebuild-able?

Thank you
Charlie T

Answer
The 1995 Discovery is an older system.  On that vehicle I'd first suggest you bleed the brakes using the somewhat complex procedure in the factory workshop manual.   If the pedal still sinks in hot weather I'd swap the ABS unit.  There is no ready way for you to test it, and I suspect you can get a unit from a salvage yard for very small money.