Master brake cylinder

[Wudy]

Yesterday I installed a brand new master brake cylinder in our 1970 Traveller. After bleeding the system brakes blocked. After many hours of cold garage floor I decided to remove the new cylinder and check. I found that the dished washer is not there, valve washer is round but does not have the dents/cutoffs around its circumference as seen from the workshop manual figure AND the second hole (smaller one towards the opposite end of the push rod)connecting the fluid reservoir with the (piston assembly) cylinder is not drilled through! Although the brake pedal comes back I have a feeling that all or one of these observations I made is blocking the brakes. By the way, I overhauled the front disk brakes (incl. pistons)and the drums in the back. Appreciate any help.

[taupe]

Hi

I think the seals in the m cyl for a disk brake setup need to be different from standard.

You should compare with whats in your old m cyl??

Im sure someone who knows will be along soon

[MarkyB]

Not really sure what "blocking the brakes" means, but here goes;
With discs fitted you need to remove or disable the "top hat" seal right at the front of the master cylinder.

I wouldn't worry about the look of the seals, it's the function that counts.

This may be the case with the missing hole, do you mean the one you can see looking into the filler hole?

Pictures, as ever, speak thousands of words.

[IslipMinor]

The seal to remove is the one ringed as 'Valve Cup'. It maintains a small residual line pressure in the brake system, which is definitely not required with discs! I doubt that this is all your problem as it only holds the pads quite lightly against the disc, enough to generate quite a lot of heat, but not enough to 'block the brakes'. It does need removing when fitting disc brakes to a Minor.

Have you got any free play on the pedal and pushrod before it moves the master cylinder piston? This is the most likely source of 'blocked brakes'. This should be checked with the transmission tunnel in place (not the master cylinder cover) as replacing the tunnel will position the brake and clutch pedals in their uppermost position, which may not be where they were when you adjusted them.

If you remove the filler cap and gently press the brake pedal whilst looking into the master cylinder, you should see a movement of fluid up from the small hole that you can see directly under the filler hole. This is normal and shows that the secondary seal is back far enough when the pedal is released, and so the primary seal as well - assuming that the two holes are drilled in the master cylinder, which would be most likely. Don't press the pedal quickly with the filler cap removed, as the fluid will spurt out of the master cylinder and dribble down into the chassis leg where you can't remove it and it will strip the paint wherever it touches.

Adjusting the pushrod free play means taking the m/c out, so make sure that you remove the 'Valve Cup' at the same time.

Let us know what you find?

[frame][/frame]

[Wudy]

Thank you very much "taupe", "MarkyB" and "Islip Minor". With transmission tunnel in I adjusted the pedal so that it moves half inch before there is contact with push rod, so that is not the problem. I also found out that the small hole in the cylinder is drilled through, I checked it with a needle, maybe there was dirt (I did notice the new m s was not very clean when I opened it, now it is sparkling). You found the problem, I am sure; I have to take out the valve cup. One question: as I mentioned before I do not have the dished washer behind the master cup. On my new m s master cup is attached to the extension of the piston without the washer. I still have the old m s and that has an unattached master cup with a dished washer. Should I leave the new m s the way it is or should I put the old washer between master cup and the piston end in the new cylinder?
Cheers to all,
Wudy

[katy]

Leave it w/out the dished washer, and see how it works.

[bmcecosse]

The Manual gets v excited that the dished washer must be in the right way round - so I'm guessing it's important!

[MarkyB]

There are holes in the piston behind it so leaving it out will mean they are blocked by the master cup.
I'd say fit it.

[Wudy]

Dished washer in, valve cup out and the problem is solved. As a new member I am impressed with speedy and good advice. My wife has the Traveller since 1993 and we never had trouble except once I had to renew the clutch, not as easy as changing my TR3 A s.
Special thanks for Katy's wisdom.

[IslipMinor]

Wudy,

There is a note in bold print in the BMC Workshop Manual that says:

NOTE: Later models have a thin dished copper washer between the end of the piston and the master cup which should be fitted with its convex side against the head of the piston. If this disc is found to be missing when dismantling takes place it is imperative to fit one on reassembly.

Sounds like you have done the right thing!