Brake Calculations

[AshG]

Ok Guys im tring to get my head around some brake calculations.

my brake set-up was this below
master cylinder 22.2mm
front calipers 2x 53.9
rear cylinders 2x 20.6

I have since changed the setup to
master cylinder 20.6mm
front calipers 2x 53.9
rear cylinders 2x 20.6

since the change i have noticed that the pedal has quite a bit more travel. the problem is if i push with all my strength i can almost get the brake pedal to hit the bulk head.

there is no air in the system and the wheels will lock at about half way towards the bulk head. im just worried that i can push the pedal well beyond the point where the wheels lock up.

my iva is approaching soon and i wouldn't want to fail because of this.

should i just get a longer push rod or should i be looking for a different solution?

[flyerncle]

Is there any chance that the front seal nearest the brake pedal is bypassing fluid and loosing one circuit.

[Big Vern]

Hi Ash,
From your figures I've calculated the change in piston movement relative to the change in csa of the M/C. you should only be experiencing an increase in pedal movement (assuming std book pedals) of about an extra 1/2 an inch over the original M/C.
What free play is there? (should be nil)
Does the pedal move then stop, then continue to creep when you press really hard?
If you need a longer pushrod then that soulds like excesive free play.

Brake systems with the M/C's below the wheel cylinders are notoriously difficult to bleed so you could well still have trapped air even though you think it's all out. did you use a pressure bleed to 'push' fluid through the system or just rely on the old fashioned 'pumping with your foot' method?

Is it a new M/C or a used unit. If the used then I'm with Flyerncle on this one and would suggest dodgy piston seal.

HTH BV.

[spud69]

Same happened on my stock sierra one, front seal started weeping. Had to pinch Handys (he doesn't need it for now...;) ) then replace his.


AndyH

[dogwood]

Ash,
If you PM me tomorrow morning to remind me.
No point doing it tonight, coz I'll only forget:rolleyes:

I'll take the one off my car. (I know that one is ok.)
And I'll fetch it back with me, just incase.

[HandyAndy]

Ash,

before you swap with Davids M/C...............try the broom stick method.:eek: :D trust me it works ..............

wedge a broom stick/ long length of steel onto the brake pedal so that the pedal is under constant pressure (hard) & leave it over night,

this will probably solve the problem , basically it gets rid of all the tiniest air bubbles back to the M/c.

on the race bikes we used to use a cable tie on the front brake lever overnight, hey presto.....brakes that would work & have a "nice feel" to them.

Try it, nothing to lose, don,t even need to get the spanners out ;) :D

cheers
andy

[AshG]

cheers dave has to be worth a try.

the one on the car is brand new. i have tried normal bleeding, pressure bleeding with an ezy bleed and i have even tried a professional vacuum bleeder.

im pretty sure that there isnt air in the system. the car stops fine but the brake pedal just doesnt feel like what i would describe as normal.

im getting to the point where i feel like ripping the whole system out and starting again

[HandyAndy]

Ash,

try the method i posted as above, ;)
with what you are describing..."there is air in your system"

cheers
andy

[AshG]

tried that one andy :D

[HandyAndy]

tried that one andy :D

do it again then :rolleyes: :D

andy

[HandyAndy]

if all else has failed then i,d bet my bartering skills that you have air in the system at the nearside rear brake line,

bleed o/s/f
bleed n/s/f
bleed o/s/r
then n/s/r

& see what happens.

cheers
andy

[dogwood]

bleed o/s/f
bleed n/s/f
bleed o/s/r
then n/s/r

& see what happens.

cheers
andy

That's what I thought when I was there helping him.
Beleive me we bled the whole system over and over again.
As Ash said just dont feel right.
But if ther is Air in the system I'm Bu*****d if I could find it..:mad:

[Big Vern]

To try to help diagnose the problem further - Is the pedal spongy or springy?
Does the pedal creep after pressure is applied to the pedal and held? or is it that you just have too much travel?

BV.

[flyerncle]

Take out all the pipes and fit bleed nipples in the holes and bleed them,if the pedal is kak you will know one stage is goosed.

[AshG]

think i am going to go with flyerncle's idea. i will stick bleed nipples in the master cyl and see if i can get a hard pedal. if its still wrong then i know the master cyl is goosed. if the master cyl is not goosed then i will connect up one part of the system at a time until i find the problem

[flyerncle]

The other trick is to clamp the flexy hoses (not braided ones !) press the pedal (need's glamorous assistant )when pressurised release hoses one at a time and feel for the pedal dropping indicating air in the line.
If I remember correctly the one nearest to the pedal is rear brakes,put the bleed nipple in that and see what happens.
The pedal pushes the front cylinder into the other with fluid and is probably the cause of the problem as it's bypassing.

Best of luck.

[dogwood]

(need's glamorous assistant )
Best of luck.

Guess that'll be me then !!!:eek: :D

[flyerncle]

You or Ginge bro ! (did not want to say):p

[gingea1pom]

I am a little distance away at the mo, but I am with you and watching your progress.

Cheers Ginge

[dogwood]

Depends if he prefers age or beauty..:rolleyes:

Oh hang on that's still me..:D

[gingea1pom]

There is no nice come back to that Dave:D

Back to the brakes:cool:

Cheers Ginge

[dogwood]

There is no nice come back to that Dave

Back to the brakes

Cheers Ginge


:D :D :D :D


OK Ash I've got the MC all ready to bring back
I'll also bring my calipers and discs
Then hopefull we will be covered.

[flyerncle]

New mirror me thinks ? :D

[AshG]

cheers dave best u2u u a list as there are those other bits. im going to have a play tomorrow if its not raining.

spent all day outside today making all the wiring under the dash beautiful also had to sort out my horn. depending where the steering wheel was changed if it worked or not. took the wheel off and it was clearly obvious that the terminals were corroded. quick sand with a bit of wet and dry and problem cured.

getting all the lights on the dash working and doing what they should be doing felt absolutely fantastic. i can happily say all the electrics are done and im more than happy with them.

next job is to config the speedo. apparently according to the dash manual you just enter the wheel diameter if the sensor is on the wheel and it calculates the speed from the wheel rotations. the only problem is i have mounted mine on the prop flange.

being on the prop with a diff ratio of 3.92 means for every 3.92 turns of the prop the wheel rotates once.

so theoretically if it take the wheel diameter and divide it by 3.92 the speedo should be dead on.

[dogwood]

cheers dave best u2u u a list

Yup. better to be covered.
No probs for me.
Anything you think you might need.

[baz-r]

ash hows your brakes? sorted yet heres a few of my ideas

whats the deal with your pads etc if you have new pads on old disc you will get more travel at your pedel even with new pads on new discs
get them bedded in if you can

ajust up the rear drums as much as possible (are the shoes new)

check your discs and drums are running tru (use a dti)
i have had this loads of times on new brakes

all your calipers sliders free if your on single pots or all your pistons free on multi pots
or just not aligned corectly?

if its all bled ok (must be by now) you should start looking at lost movement

not sure with the iva but the mot rule of thumb was if the brake efficency was in well berfore the end of travel stop it was fine the old fiesta's where bad for this as thay put the master cyl on the other side with a link rod and it would twist under the strain

[AshG]

i think its sorted. i have got a funny feeling that the mc is just a little on the small side combined with un-bedded parts. took the car out for a test stop and i can lock the wheels very easily

i think the excess travel could be contributed to by the fact that the pads and shoes are not bedded in yet. i have a new MC, new discs pads and callipers on the front then new shoes and cylinders on the back. the only bit that is not new are the drums. they had a bit of a lip but i wouldnt have said it was bad.

will see what dave thinks when i comes over. i could go and get 2 new drums but it may be a waste of money.

[flyerncle]

Grind the lip off and click the adjuster's up manually,much better pedal than letting them self adjust.