Brake Question

OK, now i thought i was fairly good with brake issues, but this one has me stumped.

Always had lots of pedal travel - could always stop but it felt a little spongey - bled the system twice with no luck. Pedal always feels resistance and doesnt sink under constant pressure but will go right to the floor (good sign of air in system, but after two pressure-bleedings?) Swapped rotors for some nice new ones, new pads (needed doing, wasnt to cure this problem) and the problem became more pronounced. Last night i had to do an emergency stop from 45 and the last car length or so the pedal just hit the floor and fortunately the car stopped eventually, in a cloud of smoke (not tyre smoke - didnt slide, the ABS didnt kick in either). Checked the hoses, fluid levels, new pads and rotors, checked the hard-lines. Checked the calipers immediately after stopping and they were just warm to touch, discs were hot naturally.

Any ideas? Should i just re-bleed again and see?

J
Reply to
Coyoteboy
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You failed to mention the condition of your master cylinder...

Reply to
Hachiroku

What kind of vehicle?

Are the rear brakes drum or disc?

Did you clean the protective coating off of the rotors with brake cleaner when you replaced them?

What is your history with the vehicle? Have you owned it since new? Acquired used? Have you check the master cylinder pushrod adjustment?

Reply to
Ray O

Thats because im not sure on it. The pedal doesnt sink with constant pressure, and even on excessive travel it always forces back with the same force i pressed down with hence i assume MC seals are ok. Servo assembly holds vacuum flawlessly, hose is in perfect condition, as is the one way valve in it.

J
Reply to
Coyoteboy

Hehe, so many questions...let me try to answer completely:

Its a '91 Celica Alltrac, vented discs up front, solid rears. Had the car for 18 months, brakes were initially very good (but anything would feel that way coming from a Peugeot 205), gradually got worse and i noticed the pedal travel becoming spongey and increasing. This was when i first bled it, before the new rotors were fitted. This didnt cure it but i needed new rotors 6 months later (here we are today) so they have just been done.

Rotors were cleaned and all mating surfaces were cleaned of junk with fine sandpaper. This weekend it was bled again, from calipers back to MC (no change) and back the other way - pushed nearly a litre of fluid each way. Bled the ABS actuator block by cracking each connecting pipe

- got a couple of squits of air out of it which improved the initial pedal travel noticably.

Now with the servo vacuum detatched or the engine off the brakes travel

1 inch then firm up nicely and i cant force it past half way with all my leg strength (you can see the pedal arm flex instead), still feels a little flexy but not spongey. But with the vacuum *on* I can still hit the pedal on the floor with a decent bit of force (read as *the force needed to do an emergency stop*).

I've felt round all the flexi hoses - you can feel them 'stiffen' on brake application but they certainly dont appear to balloon at all. All the hard lines were replaced 12 months ago due to excessive corrosion on one back corner of one line. At the time it didnt make a difference to the pedal feel so i assume it was bled out fine then (or at least that hasnt been a contributing factor).

It seems entirely linked to the servo assist unit - with it off it behaves as normal, with it on it feels like the pedal arm is made of spring steel. I dont think its the pushrod adjustment - the brakes come on pretty early - a good bite occuring at about 1 inch, noticable retardation earlier than that. If only it would stay somewhere near that one inch mark on firmer application id be happy with them, but it doesnt and it doesnt seem to exhert any more force on the pads.

J
Reply to
Coyoteboy

I think you're on the correct track - check out the vacuum booster (servo assist unit).

Reply to
Ray O

I dont know any clear way of checking it to be honest. The vacuum uni holds vacuum perfectly well, the one-way valve in the hose works fine - i cant think of any other test i can run on it :(

A slight alteration to the pedal feel i noticed this morning - if i press the pedal, while driving, once rapidly i can feel a springy feel then a sort of 'moving damper' effect - the pedal feels as though its hit something a little more viscous, then it continues to the floor. If you press the pedal slowly it takes more travel to hit the 'more viscous' feeling bit.

I'm almost tempted to go against all my normal feelings and drop it in at toyota to be checked out, but they'll probably take the pi$$ with the bill.

J
Reply to
Coyoteboy

Are you checking the cylider or the booster. This happened to me once a long time ago, back when I used to PAY someone to fix my cars, and I can't remember which part they replaced...

I would just go to a junkyard, find a good booster/cyclinder assy and try it...

Reply to
Hachiroku

Hmmm...is that part specific to the AllTrac?

Reply to
Hachiroku

Im not sure to be honest. I do know that the alltracs from 87-98 have about 7 different part numbers with slightly different specifications for each, depending on whether they are JDM, UK, ABS or not within each country etc and have heard of problems with fitting even the JDM version to the same model UK so im guessing they are alltrac specific. Just my luck eh!

J
Reply to
Coyoteboy

About right.

My '85 Corolla was the 'new model'. Even thyough the model year started in September, they were still selling '84s! April of '85 brought the actual model change, so when I got for parts they ask, "When was it made?" April, '85. Um...OK...what week! I have to use the VIN (common practice now) because the car was made in the first week after the change.

And the parts are 2x as expensive!

Reply to
Hachiroku

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