Brake pedal sinking

[Woodyalan]

This is a car thats been off the road a long time and the brakes were not working. Finding the master cylinder was empty I filled it and bled the system and the pressure was soon restored. However now I am finding if I keep pressure on the pedal it will very gradually sink to the floor. The rest of the system is fine with no leaks. I am wondering if a repair kit will cure it or will it have to be a new MC.
Alan

[mike.perry]

It's your life on the brake pedal, I would replace the master cylinder

[simmitc]

Your life and others too, there's no question that you should overhaul and replace as required. From what you describe, yes, repalce the master cylinder. If the car has been out of commission for a long time then you may want to flush the system and replace other components too.

[minijojo]

as the others told, go the safe way, replace the MC and see what else is damaged with the brake system

[biomed32uk]

3rd'd, you need to be going over those brakes like a rash, as said if the car has stood for a long while then they need an overhaul.

No room for error with single circuit brakes, all or nothing !

Brakes and cheap don't go together either.

[James k]

As others have said, replace the master cylinder and also closely inspect all the wheel cylinders. I had the same problem when I bought my car and it was very worrying when I noticed and was on a hill! I take it you're not driving the car.

[Woodyalan]

The car is stripped down for restoration. I have now removed the MC cover plate and found it very wet in the chassis leg. I will be taking the MC out anyway for overhaul or renewal. I put it on the forum as I remember reading an article a while ago, probably in a technical manual, where someone wrote about the symptom I have described and what would cause it. I can't now find the article but I thought it might be to do with the internal seals which would have been much cheaper than a new MC.
Alan

[biomed32uk]

Definitely the seals in the MC then, fluid had to going somewhere.

You can take it apart and inspect, if the bore is perfect with no pits or blemishes then you may get away with a seal kit.

I'm of the opinion that this is the heart if the brake system, if it fails when you heave the brakes on hard then you are in trouble, at least a wheel cylinder weeping you have some chance of some stoppping.

In the scale of a restoration for peace of mind I would fit a new MC. Apart from the usual servicing I do on mine I give the brake pedal some good sustained pressure before I take her out.

[ianmack]

It is quite possible that the fault you describe is caused by poor seals. When there is pressure on the pedal fluid is pushed out of the cylinder as you have seen and possibly back to the reservoir as well. It is cheap to buy new seals and this might solve the problem, but only if the rest of the cylinder is in perfect condition. Unfortunately an old cylinder will sometimes have tiny imperceptible blemishes which result in the new seals not sealing perfectly.

In the case of the Minor this is complicated by the inaccessibility of the master cylinder, as you are about to find. This is why Minor people recommend a new cylinder rather than just seals. It would be galling to re seal a cylinder only to have to pull it out again a month later because sealing hasn't worked.

[Trickydicky]

I have had a m/c fail while driving and trust me you never want to experience it! Even using the hand brake and trying to go through the gears to slow the car is something you never want to happen again.
For the cost of a replacement m/c and the peace of mind it's a no brainer.