brake diagnose problems.

[MAV]

Gday all.

Im hoping someone can shead some light on this problem for me.
Im fixing a freinds car for rego,oils leaks,smashed tail light,general clean up,But i also had to bleed the brakes as pedal was going right to the floor,brakes still worked but only in the last half inch of pedals full travel.

I tried bleeding them,but no matter how much i try air still keeps coming thru,ive looked for leaks everywhere with no signs what so ever. and i all can think of is a stuffed master cylinder,am i correct in this thinking???

I know how to bleed brakes and have done it many times with no problems,but its driving me crazy,car still has no pedal but brakes still stop car..all pads are new,master cylinder looks fine but ive never really had problems with them before so dont know how to diagnose it.

Car is a early 90s toyota cressida(aussie car not sure if usa has them or if they are called by another name there) 4Wdiscs.

Anyone have any other tips for me on this???any help would be greatly appreciated.
cheers,
mark.

[chimp koose]

sounds like you need to bleed the master cyl.

[MAV]

sounds like you need to bleed the master cyl.

Yeah i did try that but not 100% on if i did it correctly,i bleed it by using the supply lines as if they were bleed nipples,fronts had no air ,rears had a little but not much.maybe i need to do this more??

I notice that your in saskatoon.i live in australia but i did live close to saskatoon for 5 or so years,prince albert.sask.

[wayne petty]

i would like to know... if the rear calipers have parking brake levers built in...


if so....   is there any clearance between the pads and the rotor

WHAT..

calipers with built in parking brake devices use the parking brake devices to adjust the caliper to brake pad to rotor clearance..

if you have more than a fraction of an inch... and i am talking just enough for the rotor to turn...   the retracted piston takes up all the fluid the master displaces and the pedal continues to the floor before the brakes are applied...


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bad wheel bearings can also cause this problem... as the weight of the car sitting on its wheels  causes the rotors to sit at an angle through the caliper... pushing the brake pedal.. uses extra travel for the caliper to squeeze the brake rotor until its straight...


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rotors with large wear ridges around the outside can also cause problems when new pads are installed....

the wear surface of the rotor actually moves at different speeds through the pads....  what....   take a 12 inch rotor... the inside diameter of the wear surface  is probably 8 inches..   the surface closer to the outer is moving faster in surface inches per revolution..

at 12 inches.. the surface moves 37 inches per revolution..
at 10 inches.. the surface moves 31 inches per revolution..
at 8 inches the surface moves 26 inches per revolution...

this offset in speed causes the outer area to wear more than the inner area....  causing the rotor to wear at a taper...

the brake calipers are designed to work with a parallel surface..

the brake pads are parallel when new...

if new pads are installed on worn rotors...  something is going to have to flex for the pads to come into COMPLETE contact with the rotor...

some cars are really critical on this.. where you need to machine or replace the rotors each time...


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frozen or corroded caliper slides can also cause this problem...

what...  yep...  the calipers can slide  and have to on many applications with pistons only on one side...

with a single sided piston caliper... the piston is pushed out against the inner pad... the caliper will then slide backwards to equalize the apply force between the inner pad and the outer pad...  

when improper grease is used... or no grease on the caliper slide pins... they can seize... preventing the caliper from being free to move back and forth...

i have found that dielectric tune up grease is NOT the thing to lube caliper slides or nuts and bolts...  why not use dielectric tune up grease... it has dissolved sand in it...   get it hot.. the sand becomes sand again..  taking up the clearance between your parts..   i had one brake caliper that i could not budge the caliper slide with my 20 ton press..

only use white lithium grease...  or Silglide on caliper slides or pins...

[wayne petty]

on a separate note...

is the master cylinder tank transparent so you can see the fluid level...


have somebody apply the brakes while you watch the level...

does the level rise as the pedal goes to the floor??? if so after the first squirt.. the master cylinder is probably bad...

the master displaces fluid into the brake system...   after the pistons seals are past the fill ports .. the fluid level should NOT Change...

when one side of the master is bad...  the fluid level will rise in  as the normally displaced fluid leaks back into the reservoir..

[MAV]

thanks for the help guys.lots if great info helped get it fixed.
master cylinder was stuffed.
mark.