why dont my brakes work?
Turbo Mini Forums

hello all, hope your enjoying the sun

instead of going out in my highly waxed car yesterday i decided to quickly put my new minispares 4 pots on

however i cant get them to work properly. Ive bled and bled and bled the cursed things but all i get is soft pedal when first pumped then hard the second time.

i have removed them from the car and bled them again with them raised up in the air, and also dangling from the floor but both times i still get the same result with no air coming out when bled.

there are no leaks in the system

when the pedal is pressed the level in the m/c drops slightly before raising back up.

all in all i got so annoyed with the f-in things that ive put my S setup back on, but i had the old style minispares 4 pots that worked fine on my car(except for the porous casting)

can any one help? im thinking there is air still in the system but it wont shift and im NOT using one of those crappy bleeding devices that you connect up to a tyre before covering the contents of your garage with brake fluid

or

if you cant help to you want to buy a set of 7.5 minispares alloy calipers, non vented and the latest type that are machined instead of cast, ?250, (?295 new)

erm, sounds silly but.

Do they have 3 nips on them like metty four pots?

If so make sure the 3rd nip is at the top to let the air out of the top pair as i have seen someone fit them with the 3rd one at the bottom and wonder why his brakes were not working.

sounds a daft question did you bleed the whole brake setup? i've had a similar result before but after bleeding all 4 corners in the right order the pedal was alot better!

start with the furthest brake from master cylinder clamp off the 3 flex hoses of the brakes your not bleeding at that time (if you have rubber not braided )

i only have 2 nipples (thank god), both at the top!!

ive tried doing all 4 starting at rear pass side, rear drivers, front pass, front drivers

ive done the front nipples both at the same time (on the same caliper) and 1 at a time

unforunately im braided all round, when i put the S calipers back on they were ok first time which leads me to believe that it must be air in the caliper

handbrake off, handbrake on, bias valve right out, bias valve right in(, put my left leg in, left leg out, in out in out shake it all about)

all with the same effect of failure

cheers for the comments though

Edited by Nic on 1st May, 2005.

im having simlar probs with my four pots, but im thinking its the master cylinder is the problem, dont no about you greypride but im using a cooper s mc, and im thinking that the internal piston bore size in the mc is not big enough so this will give a greater brake travel.

but am i barking up the wrong tree here

always looking for them bigger bunches of bannanas

if you a`re "SURE" you have got all the air out!!
IF SO.............................!

change the master cylinder this is where the problem lies....................

i think fitting 4 pots has shown up a weakness in the master cylindar

(i bet it older than me ?)

arund 40 quid...ish

keep us informed

A new master cylinder is £100 I think from Minispares.
Still a good price for peace of mind I think.

edit: Just checked: http://www.minispares.com/Product.aspx?pid=36659

Edited by Vegard on 2nd May, 2005.

I know you aren't keen on a pressure bleeding kit but why not? If you use an old wheel and tyre and let it down to around 20psi before you start then you don't cover everything in brake fluid?
I use one of these regularly on all sorts of cars and it always bleeds stuff up first time no problems?

I always get this bloody wrong.......I'll work it out for me as much as anyone else.......

If caliper total area is 4sq", and master cylinder is 0.75sq" (simplified) then 50lbs force on 0.75" at the master cyl equates to 266.6lbs force at the caliper. (Pressure = Force/Area)
(Pressure created at Master Cyl = (50/0.75) = 66.6lb/sqin.

At caliper end, braking force = Line Pressure*Area = (66.6*4) = 266.6lbs Force)

If master cyl size is 1.5sq" (twice the above) then force at the caliper drops to 133.3lbs.

Therefore force with a master cylinder twice the size required to give the same caliper force = twice as much, e.g. 100lbs required with the larger master cyl to give the same 266.6lbs at the calipers.

Its easy to get confused with pressure and flow. I do it as much as anyone else, you can go round in circles. A "nmemonic" I was taught when studying industrial hydraulics is "pressure does the work, flow makes it go".

In a braking system, it is the pressure that does the stopping, the flow merely pushes the pads the very small distance into the brake disc to allow pressure to build up and "do the work".

its a new master cylinder, the plastic bean can style

ive used it with my old minispares 4pots and it worked fine, and it works fine with the brakes that are on there at the moment

ive had such disaterous results with the pressure bleeding kit in the past that im not even going to take it out of the box, all it successfully did was spray fluid over the car

grrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr

(but thanks for explaining mc sizes to me)

I had a problem with a set of 4 pots once, the drillings between the pots on one side of the caliper had wandered during manufacture, meaning that the fluid was effectively blocked off by the seal. When trying to pump the pedal, it was very spongy, and one piston refused to move full stop. When you press the pedal do all the pistons move?

A long shot I know but worth checking?

thanks for persisting

ill try that next weekend (if i get chance)

Give you £100 for them...

Ruthless f***er!!!

Nic, can you not throw them back at Minispares?

i would shop around for the master cylinder

i said 40 quid ish.....

but that was 5 years ago

i bet you can maybe get one for 65ish

shop around

theyre my second set from them, the first were the cast ones that leaked fluid

i think i will get in touch with them again-i just didnt want to get intouch when i had missed an obvious trick, if not ill sell them and buy a kad set or similar

nevermind the running in is nearly finished!!

Just replaced complete pipes cylinders servo on a golf,bled 4 times no pedel......got a brand new easy bleed kit and perfect.
Think easy bleed kit is ok when new but the rubbers and pipes go hard when left I'm on about my fourth one now.

You may need smaller rear cylinders if you change the front as the master may not put enougth out to move the lot ???

Metro turbo weekend driver,Mini turbo in the making again!

had the same over the years the pipes perished

on the kit, i always use the traditiaonal method from some 40 years ago

you get brake fluid all over but does the job !!

use the kit once and then change the pipe

brake fluid is evil and will stip & erode most man made items.

hmmmmm, i may possibly consider buying another then, but im not keen

grrr moving house at the mo such not much time for mini tinkering this weekend

the best one ive seen is a 99p pearl which is a piece of pipe with a split in it with a stud at the end works a bloody treat and my last one ive had for years.

always looking for them bigger bunches of bannanas

I bought one of those once. Except mine didn't have a split in it, so as soon as you pressed the pedal it shot the stud out never to be seen again. I couldn't figure out how it was supposed to work so I had a strop and set it on fire.

oh hang on, are you talking about the thing that you connect up to a tyre to bleed, or the tubes that you slip on your nipples and only allow air and fluid to go out, but nothing to go back in??

if the latter thats what ive been using