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

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. 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

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

[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.