master cyl mod

Post by **les** » Wed Jan 03, 2007 9:30 pm

I'm getting ready for a disc brake set up, and about to remove the small seal from the master cylinder. Remembering a post a while back explaining why this is needed, it had something to do with preventing the pads from holding on slightly, the seal being needed with drum brakes to prevent fluid from draining back? Well fair enough for the disc set up, but as the rear still has drums I'm thinking the original set up is then needed. I can't remember who posted this info, but would like clarification on this combination.

Post by **Kevin** » Wed Jan 03, 2007 9:36 pm

Well fair enough for the disc set up, but as the rear still has drums I'm thinking the original set up is then needed.

But shoes have springs to pull them back pads dont, the other alternative that is reccommended by some is to drill a hole in the seal I think around 1/8" to allow the pads to ease back.

Post by **bmcecosse** » Wed Jan 03, 2007 9:58 pm

It's a lot of work - why don't you just try it as it is and see if it's ok without butchering the master cylinder.

Post by **les** » Wed Jan 03, 2007 10:01 pm

But shoes have springs to pull them back

Kevin, I understand that, but I thought the seal maintained a bit of pressure to keep the fluid from leaving the wheel cylinders, what stops this happening when it's removed?

Post by **IslipMinor** » Wed Jan 03, 2007 10:04 pm

The valve in the master cylinder is there to maintain a low level of residual line pressure in the all drum braked system. With discs fitted, this results in the pads remaining in contact with the discs, and getting VERY hot! Just ask the local tyre fitter who took the wheel nuts off! I knew, but had fogotten to remove the seal when fitting a new master cylinder.

I also wonder whether is has anything to do with the under-floor mounted m/cyl? I have the remote m/cyl, mounted up on the inner wing, so if it did this will overcome it.

It doesn't matter whether it is removed or 'immobilised', but it shouldn't remain working, or pads will not last long, and a lot of heat will be generated that does the front hub grease seal no good at all.

Post by **bmcecosse** » Wed Jan 03, 2007 10:04 pm

The springs pull the shoes back - and the pedal travel increases unless you keep the shoes very well adjusted up to the drums. The front pads should remain close to the discs anyway - unless there is wear in the hub bearings and/or run-out on the discs. If you leave the little seal in place - the pads 'may' rub slightly on the discs - but i suggest you try it and see what happens in practice.

Post by **les** » Wed Jan 03, 2007 10:14 pm

Thanks, most people seem to suggest removal, but as I keep trying to say, the rear brakes are STILL drums so if a seal is fitted for that reason what happens to the rear if the seal is removed for the benefit of front discs? Maybe I will try without removing it.

Post by **Kevin** » Wed Jan 03, 2007 10:26 pm

As most of the breaking is done by the fronts I would have thought that the slight loss of pressure to the rear would not be noticeable, but dragging pads would not be a good idea.
Les I think the best person to ask is Jonathon he should be able to give you the definitive engineering reason for it.

Post by **les** » Wed Jan 03, 2007 10:34 pm

Thanks Kevin, tend to think you are right about the slight pressure loss, just found the owen burton book and according to that, the pressure is about 5lbs, and it also said the pressure is there to keep the seals expanded against the wheel cylinder walls, removal of this seal only requires a different bleeding method, again reading from the book.

Post by **jonathon** » Wed Jan 03, 2007 10:47 pm

Les, definately remove the top hat seal or drill a 1/16 hole in the bonded rubber. If you leave this seal in you will overheat the pads and discs and run the chance of either a lock up , or brake failure. We have employed this method for 13 years on both other kits and our own with no failures or problems. Offering advice when not in full posession of the facts is poor form, so please do listen to the 'informed' advice, unless you feel the desire to be a crash test dummy.

Post by **bmcecosse** » Thu Jan 04, 2007 7:51 am

My advice was to wait and see - that's all. But the fact is - as Les has pointed out - the rear seals are designed to be held in place by that slight back pressure - and this will not now be there if the master cylinder is modified. Modern car brake cylinders are designed with very different seals which work without a back pressure. I would at least fit new rear cylinders to have the best chance of them working under the new conditions.
I also wonder how the insurance co reacts when you tell them you decided the design of the master cylinder is wrong - and so you drilled a hole in one of the seals !!

Post by **jonathon** » Thu Jan 04, 2007 8:30 am

'but i suggest you try it and see what happens in practice'. don't really think this is quite the same as 'wait and see'. The drums will work fine with the mastercylinder properly 'modified' not 'butchered'. The insurance companies are well aware of these modifications as they have been around for nearly 30 years.

Post by **bmcecosse** » Thu Jan 04, 2007 1:10 pm

Yes - 'try it and see what happens' is 'wait and see'.

Post by **hubcap** » Thu Jan 04, 2007 9:07 pm

Do NOT under any cirsumstances "try it" or "wait and see", your life depends on the brakes working properly. From experience I can tell you that leaving the seal in place WILL lock the front wheels. Removing it WILL NOT stop the rear brakes working. Follow the professional advice and remove the seal. (make sure that it's the correct seal!!) If in doubt take it to an expert, don't take a chance.

Post by **Nadir** » Thu Jan 04, 2007 10:46 pm

Seems simple - all the people who make the conversion kits recommend it, and all are fully liable for the kits that they make and the instructions that they give. You could follow the wait and see advice, but it might well kill you.

Post by **les** » Fri Jan 05, 2007 9:04 pm

Seems simple

Exactly, but I have a habit of wondering about things, and the rear brake query was one of them. However the posts received have pretty well convinced me to remove the seal, the hole in the seal bit I think I'll give a miss.

Post by **bmcecosse** » Fri Jan 05, 2007 9:28 pm

No - it won't 'kill' you - my idea is to wait and see if the brakes drag with the little seal still in place. Obviously if they do drag - then it will have to be removed. But some calipers have fairly stiff piston seals - and they may well be fine with the seal still in place. It's quite a job to remove the master cylinder, strip it down - may as well fit a new set of seals while in there, and then refit it to the car. There is a much easier way to get excellent brakes on a Minor!!

Post by **les** » Fri Jan 05, 2007 9:47 pm

The cylinder is new and awaiting installation so to avoid removing later I need to decide now, which I have done.
I bet you're thinking of your flipping 8inch drum brakes !!! Oh you're so yesterday!

Post by **Onne** » Fri Jan 05, 2007 9:51 pm

9" Les !

:D:D

Post by **bmcecosse** » Fri Jan 05, 2007 9:55 pm

Yes - 8 " are far too small. Not so bad you have the cylinder in front of you Les - in that case i too would remove the unwanted seal before fitting the cylinder. That wasn't clear at the start!!