Binding Brakes

[Minornut59]

Hi.
I have had a problem with my Minor brakes and wonder what others think.
Earlier this year i had my master cylinder fail so took it out, rekitted it, and replaced it and all was well for a bit until i noticed the brakes binding.
I realised this was the master cylinder and in the end to save messing about, replaced it with a brand new Lockheed one.
All seemed well until i realised that the brakes were still binding but not so much that the car would grind to a halt as it did before.
I took the wheel cylinders apart and cleaned them up, and freed them as they were a bit sticky, reset the pedal clearance, (which is now more than it should be) and that was Ok but moving the car in and out of the garage yesterday the wheels were squeaking and sure enough the brakes are binding again.
Ive jacked up, checked the wheel cylinders, which are fine, replaced the hoses as they were a bit old, and the problem persists.
The wheels will spin freely until you apply the footbrake and then the wheel will only just turn. It doesn't matter where you put the adjusters, they will still lock after applying the brake.
After a half hour, the brakes will have freed off and the wheels will turn.

Its the master cylinder isn't it?

But why, as its a new one????

Thoughts please....

[bmcecosse]

The master cylinder is designed to hold a slight residual pressure in the system after application - to keep the shoes 'close' to the drums. You say it rolls freely after a little time - as the pressure sags away. So - I think what you have is acceptable ! Worth checking if all the return springs are in decent condition - and fitted correctly.

[Minornut59]

Hmm, I'm used to Minors, having played with them for 30-odd years and this doesn't seem right to me.
It will hold the brakes so that the wheels get hot, and that can't be right, can it??

It could be that the 'new' master needs to settle down i suppose but the resistance on the brakes is noticeable, especially comapred tothe other two roadworthy Minors, one of which had a new master last year.

ive just been playing with it, and i have had the drivers front wheel off, and experimented with pressing the pedal and then checking the drum and it will hardly turn afterwards. Leave it for an hour and it will feel easier.

I think i'll just have to try it and see out there on the road and put a few miles on it.

[MarkyB]

Tricky one, is the pedal return spring present and attached at both ends, does the pedal move freely on the shaft also is the breather hole in the M/C cap open?
Can't think of any more at the moment

[les]

Take a rear brake drum off and apply brakes, obviously not heavily but gently to open the cylinder, release then see if the piston fully returns. I would suspect sluggish wheel cylinders.

[bmcecosse]

Yes - me too for 'sluggish' cylinders... The brakes should not be held on to the extent of getting hot.. assuming the return springs are correct (are they?) - then perhaps you are adjusting up the brakes too tightly. Slacken back every adjuster by one click - how is it now?? You could of course open up the cylinder and remove the 'top hat' seal - as is done when using disc brakes - but this should not be necessary.

[Minornut59]

Well, i suspected sluggish wheel cylinders but i don't think it is that since i checked the front ones both sides, took the pistons out, carefully 'wet and dry' sanded the bores, washed with clean brake fluid and replaced. No difference.

If you have the front brakes completely de-adjusted the weel will spin freely until you gently apply the footbrake and then the wheel is instantly tight. If this were a case of a sticky clyinder then that wheel alone would get hot, but they all seem about the same. they don't bind and lock on but just get warm and there is 'drag'.

I replaced the hoses as i wondered if they were collapsing internally but no.

I'm sure its the master.

The cap is a new one with the new cylinder and the breather hole is fine.

Also, with the cap off, when you gently press the pedal by hand, a small fountain of fluid squirts up, which i seem to recall is normal.

The pedal clearance should be 3/4 in but mine is now 1 1/4 as i made sure it had plenty.

Any other thoughts??

[bmcecosse]

No silly servo - I hope.... Have to say - it 'must' be in the master cylinder then - all been suggested already - but inspect return spring and free play on the pushrod into the master. If all ok - remove the top hat seal, nothing to lose really by trying that.

[Minornut59]

Just took it for a good ten mile run in the rain and all seemed fine except again the brakes seem a bit sticky sometimes and the car seem to be struggling slightly.
I don't really want to bugger around taking the master cylinder out again right now so i think i'm just going to drive it and see what happens!!

[mike.perry]

If the back brakes are binding it could be that the handbrake cables are rusty/frayed and preventing the pistons from returning

[lowride stepside]

Are you sure the master cylinder is retuning fully , push rod not to tight ,as I had this exact prob after fitting a new master cylinder on my old lady's car

[Minornut59]

Thats an idea. I'll take the inspection cover off and jiggle the pushrod to see if its a tight fit in the cylinder. there splenty of free travel there though..

Not solely the back brakes either as all wheels are warm after a run.

[bmcecosse]

But all my wheels are warm after a run too - using the brakes makes heat! I think you are over worrying this one.....

[irmscher]

There has been a lot of bad new master cylinders on the market leaking seals etc

[bmcecosse]

Possibility of course is that there is an over strong spring inside the cylinder - causing the retained pressure to be too high. Has anyone ever measured the normal retained pressure - said to be 5 psi or so I believe??

[chesney]

Not possible one of the copper brake lines has been damaged on removal/assembly? Could be holding pressure on the cylinders?

[philthehill]

According to the BMC Wksp Manual Section M the residual line pressure (on a standard car) is approximately 8 lb/sq inch and which is required to keep the wheel cylinder brake seals expanded.

[bmcecosse]

But has anyone measured it??

[Minornut59]

With respect bmc-eccosse, ive owned this car for 27 years and i know it like the back of my hand. i know every rattle, every squeak, and how it should perform. It just doesn't feel right...

I fully appreciate that brakes depend on friction which generates heat. Normal warmth from the hubs is acceptable.
This doesn't feel normal.
I drove it again tonight and this was a ten mile run through twisty windy single trach roads, up and down hills, so frequent brake dabs.
When i got to my destination i immediately checked how hot the hubcaps were and the front ones were hot, the rears not very.
the symptom is like someone had over adjusted the brakes; a very hard pedal.
As i said before, completely de-adjusting the brakes should make for a lousy pedal but there is NO DIFFERENCE, ie' a very hard pedal. That is not right.
And there is still more than the requisite amount of free travel on the pedal...
When i got to my destination the front wheels were going squeak as they were binding only ever so slightly, but enough to give rolling resistance.

[bmcecosse]

Well - it is as several have pointed out to you - too high residual pressure in the brake system. So - either try removing the top hat seal, or fit a different mastercylinder. One thing you could try - look at where the return springs fit on the front shoes - there may be other holes on the shoes (perhaps depending on make) further apart (if not - you could drill them!!) where you can hook the springs - and this will pull the shoes back together more effectively - so if the problem is mainly at the front, this may make enough of a difference to cure the problem. Worth a try I think.