50 replies to this topic

[garysmoc]

I have 1979 mini 850 drums all round. It has always suffered excessive pedal travel. I have replaced back plates and cylinders, shoes, drums, master cylinder, bulkhead valve. It has been in a mini specialist many times and after using a hydraulic bleeder finally got pedal to just above halfway. After driving just 50 miles, pedal was hitting floor, pumping them gave some weak stopping power. I have removed all drums and checked no leaks, readjusted so light binding, Re bled through and found no air. I have checked every joint for leaks, although master cylinder level never drops, I do not think its a leak. I have checked flexible hoses and do not appear to be ballooning and externally are in good condition.

I've had mini's for over 25 years and I have never had a problem so hard to solve. The car is just 13000 miles from new, absolutely original and never modified in any way.

[sledgehammer]

you have done most of the things I would have checked - is it any better with the handbrake on ? (would indicate rear brakes if better)

 

another thing to check is to clamp all the flex pipes (all 4 wheels) , is the pedal still low ? - this eliminates the slave side of the system

 

if there is an improvement then check each wheel to eliminate it

 

also check for slack in pedal & mech leading to the master cyl

 

when adjusting the adjusters , pump the brake a few times (drum on) , spin the drum & adjust again - helps to position the shoes

 

best of luck

 

edit - all the above assuming no servo ?

 

cleverer people will be along soon with other idea's - everything above is just my wild guess

 

another remote possibility - are the rear cylinders the correct size -

 

larger ones will need a bit more pedal movement , which may make the pedal slightly lower - maybe

Edited by sledgehammer, 23 May 2015 - 10:41 AM.

[Cooperman]

As 'sledgehammer' suggests, clamp each flexible hose in turn, using the correct clamping tool (or make one up yourself with two half round pieces of wood and some Mole Grips). Then try the brake pedal doing one hose at a time. If the pedal is 'soft' on all of them, then the problem is most likely to be at the master cylinder end. i.e. before the slave cylinders and brake shoes.

Drums can be difficult to get the air out of.

One suggestion, when doing the rears, make absolutely sure the bleed screw on the side being adjusted is fully open before pushing the pedal dow. If it is done with any sort of pressure in the system the relief valve can 'kick in' and isolate the rears completely. I learned this many years ago when I had a similar issue with a Mk.1 998 Cooper.

[garysmoc]

Thanks sledgehammer and cooperman, I was at garage and watched them put a block in the splitter to the rear brakes to isolate them and check it wasn't the fronts. This suggested the rears were not effective, so most effort was put into bleeding rears. The garage has a hydraulic bleeder which they fit on bleed nipple, this found loads of air that just wasn't visible before.

I will try the adjustment technique suggested and see effect and then try clamping hoses.

There is no servo. The rear cylinders are the same external size at the old back plates had on them. The bore size could be different but I think this may be unlikely.

[sledgehammer]

Apparently there are 6 different types of rear wheel cyl    - I knew of 5/8 , 3/4 , 1/2 , but there are more -

 

http://www.ukclassic...eelcylinder.asp

 

I agree with cooperman on the rear bleed nipple at the bottom of the cyl makes it hard to get the air out - also bleed it slowly 

 

maybe try the fitting above it to get the air out - you will get fluid over the back plate , but it may just work

 

I have in the past bleed unions to get awkward air out , esp in systems that have to be bled slowly

Edited by sledgehammer, 23 May 2015 - 02:03 PM.

[Stu.]

Sounds like air to me. Try using a Gunsons Easybleed kit. I had the same problems on my 79 850 and pittled about with it for 2 weeks trying to bleed it and thinking it was something else.

 

It took 30 minutes to bleed the full system with the Easybleed an it sorted it immediately. I wouldn't bleed brakes / clutch with anything else now, its so simple and effective to use.   

[garysmoc]

Thanks All, I will try all the suggestions and update on results

[Ivor Badger]

Which direction are you turning the front adjusters to set the front shoes? are the clips an the front shoes to the wheel cylinders?

[garysmoc]

In the forward direction of the wheel and yes they the shoes are correctly positioned and springs in right place.

[Ethel]

Failed master cylinder piston seal can cause this, could also be the brake proportioning valve. Do you know which pattern of valve and master cylinder you have - fam7821, yellow or black band?

[Ivor Badger]

Sounds like one side of the twin circuit MC has failed, but you say you have replaced it. Common fault on minis at the time when the car got old and the driver would drive around with a long pedal and 2 brakes.

[garysmoc]

The master cylinder is genuine ap one from http://minispares.co...|Back to search yellow tag. The symptoms were identical before changing this one week ago. It is a yellow tag.

I have today used a brake clamp tool To clamp each hose individually and checked pedal as each was fitted, this made no difference. I pumped brakes and adjusted every adjuster do brakes binding and slackened slightly, no difference to pedal.

I bought and used a brand new easy bleed and bled master cylinder as gunsons suggested on mini's and then every Union, checking pedal after each was bled. I bled though around 1 litre of fluid. The easy bleed did not on a correctly inflated tyre have pressure on own to get fluid through rears brakes, pressing pedal revelealed some pressure but after pumping it seems to flow better. After bleeding every on, pedal still hits floor and if pumped comes back up a third and one released it then will hit floor again.

The wheel cylinders fitted are correct ones according to mini spare website.

[Ivor Badger]

Wrong length of pushrod from pedal to MC?

[garysmoc]

Wood and Pickett suggested this but do they come in different lengths and how would you change it? Last week did managed to get pedal to 2/3rds and it gradually got worse as they were used. Braking power was really strong and now is really feeble even when pumped. Found the car had rollpin missing and a loose bolt in its place. Not sure how pedal was holding on. This has been corrected.

[sledgehammer]

has the bolt worn the yolk , enough to cause slack ?

 

i'm wondering if the valve at the back is restricting the fluid enough to stop it bleeding the rear cylinders