Braking News

[Myrtles Man]

In a recent episode of Wheeler Dealers they bought a 1965 850 Mini that had lain unused for twenty odd years and carried out a light restoration to make it properly fit for use in today's traffic conditions. One of the upgrades that Elvis (Mike Brewer's current mechanical wonder-worker) treated the Mini to was a conversion to front disc brakes. In detailing the work involved in the conversion, no mention was made of a 'top-hat' seal removal as would be required in the equivalent upgrade on Minor brakes. Was this an oversight or was the early Mini drum brake set-up different from that of the concurrent Minor range? Anyone care to enlighten me?

Incidentally, the conversion to discs was probably not a bad idea as Elvis also carried out some nifty head work, resulting in a top-speed capability of 90mph! Pretty impressive for an 850.

[geoberni]

Single Circuit Classic Mini MC looked a lot different to the Minor one.
Perhaps it never had a Top Hat?
http://www.minispares.com/product/class ... GINAL.aspx

[philthehill]

The type of rear wheel cylinder seal fitted to the Minor rear wheel cylinders requires a small amount of line pressure (8 lb/sq in) into ensure that the lip of the cup seal is firmly pressed/expanded against the wall of the wheel cylinder.
The top hat seal 'also prevents fluid pumped out from the cylinder when bleeding from returning to the cylinder thus ensuring a fresh charge being delivered at each stroke of the pedal.'
The Mini does not have the same rear wheel cylinder seal http://www.minispares.com/product/Class ... o%20search
so does not require the top hat seal.
The master cylinder does not have the top hat seal either.
http://www.minispares.com/product/Class ... o%20search
There are other Mini master cylinder repair kits but as far as I can see all without the top hat seal.
Removing the Minor master cylinder top hat seal does not appear to have a detrimental effect on the Minor rear wheel cylinder cup seals effectiveness, but to be on the safe side I have fitted Wolseley 1500 rear brakes which have a ring type of wheel cylinder seal similar to the Mini rear wheel cylinder seal.

[Myrtles Man]

Thanks for that Phil - helpful and comprehensive as always.

[jaekl]

There's also a significant difference in the layout of the braking system with the Mini master cylinder significantly higher than the wheel cylinders.

[oliver90owner]

There's also a significant difference in the layout of the braking system with the Mini master cylinder significantly higher than the wheel cylinders.

Can you explain any significance of this fact, please, wrt system operation. As I see it the very small water-gauge difference in head is insignificant, in context with the system pressures attained. The over-riding requirement, of either/both, is simply a flooded suction at the master cylinder?

[philthehill]

There should be no difference in application just because the classic Mini brake master cylinder is located higher up and operates in a vertical motion as opposed to the lower horizontally mounted brake master cylinder of the Morris Minor. All that matters is that the top of the brake fluid is higher than the piston seal(s) in either master cylinder when the brake pedal is depressed.

[jaekl]

The height difference does make a difference. Both cup seals and the type that reside in grooves in the piston are one way seals. When the fluid cools and when the ambient conditions change the brake system may be below atmospheric pressure. With a low master cylinder that would allow air to enter through the wheel cylinders and then would be trapped. This can happen with a high master cylinder but there is no tendency for the fluid to drain back to the master cylinder as it will with a low master cylinder. Also, with a high master cylinder there is always some pressure head due to the height of the MC over the wheel cylinders.