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MG TD TF 1500 - TRW master cylinder failure

I installed a new TRW MC in my TF 6 months ago. Everything performed well until these past few days when the brakes began to feel "squishy" for no reason. I did a full inspection with wire wheel drums removed. No leaking. All wheel cylinders are fairly new. My wife and I did TWO very thorough bleeding procedures this weekend. No change. Of course, there is fluid in the MC.
This is the symptom: I apply the brakes and they initially feel firm. But, with prolonged foot pressure on the pedal, it will slowly go to the floor in about 60 seconds. No fluid leaking anywhere.
I can make no other conclusion than internal failure of the master cylinder. I plan to phone the vendor tomorrow morning. I'd like to think there is some guarantee on a master cylinder.

Any other ideas?

'54 TF
T Norby

I remember TRW having a bad batch. Few years ago. I suspect you have one of the bad ones.
D. Sander

Are you using silicon brake fluid and, if so, are you using modern seals that are impervious to silicon fluid?

Lew, good thinking, but I don't think anybody is selling real rubber for brake parts, and surely not in a new TRW master. They (TRW) know people may be putting in silicone. The silicone fluid is thinner, but still a new MC should not be leaking anything past the seals. Only way of getting real rubber in a MC would be if you found some old NOS bits in usable condition. But who uses old bits to rebuild a MC? But you right, Lew, if someone had an old MC well maintained such that it had 25 year plus old rubber in there, and then switched to silicone, it could cause trouble.
Sam Coleman

No, not silicon. Sam, as I said, this was a brand new MC 6 months ago. Nothing rebuilt here.

'54 TF
T Norby

This thread was discussed between 28/08/2016 and 29/08/2016

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