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Techspy

Brakes heavily biased towards the rear?

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Tried a few search terms but nothing popped up.

1994 s4 all stock brakes (non Brembo)

- Clue one: Rear wheels quickly covered in brake dust, fronts not.

- Clue two: On my 3rd set of rear pads without having to replace the front and my last rear set didn't get many miles.

- Clue three: Pulling into the car wash and spaying the car down, steam and sizzle from the rear rotors and none from the front (Not intentionally spraying the rotors by the way).

So, is the only possible issue here needing to bleed the front brakes? My brake pedal and braking feel is fine but not sure if air in the front will affect the rear or cause spongyness on this brake system.

Thanks for any info.

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That makes sense. However when changing the pads, the calipers seemed fine and retracted with no issue to fit the new ones. As far as I can tell they both are experiencing the same issue. As far as the parking brake, when I had the calipers off, I recall the cables on the calipers being loose enough for the cable to move slightly on the arm. Meaning the cables were not tight and holding pressure on the lever.

Thanks

EDIT: To add some info, I have always found it a PITA to retract the pistons when changing pads. I loosen the bleed valve and rotate and press the pistons to get them to go in. I have no idea how easy or loose these should be but do notice that when I unbolt the caliper to replace the pads, the pads are never pressed against the rotors like the pistons are stuck etc.

Edited by Techspy
added

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You have Brembo or Toyota front brakes?

The rear Brembo caliper often freezes up and the pistons actually crack... 

Remove the handbrake cable from the arm in the rear caliper, and then try to see if the rear brakes are still heating up. and check if the car will roll easily enough with no rear handbrakes.

I would also assume that if your rear brakes are getting that hot, then the fluid would also eventually boil and the pedal should get soft. The fluid should be blackened near the rear calipers.

If the handbrake is not sticking and the caliper is not sticking then I would guess that the Master cylinder spool valve for the rear brake circuit is malfunctioning, or the and the pressure is not being released.  it could be that the brake pedal is not coming all the way up and keeping the spool valve open, allowing the high pressure fluid to push the rear calipers.  Try pulling up on the brake pedal. Or check you pedal adjustment (Section JF) to ensure the pedal can return all the way

Some say that the brake hoses can collapse and hold the fluid pressure as a one way valve... not sure I buy that.

Espritmon can read ABS codes (have to connect to the ABS computer plug under the dash IIRC) and then you can also activate solenoids and see if any aren't working or are sticky.

The Delco ABS manual inside the Esprit shop manual is very helpful! It is section JF in the 93-V8 manual.

There are solenoid valves, one for each front brake circuit, and one for the rear. They modulate pressure (pressure hold or pressure release). Only used during ABS activation.
There are Displacement cylinders, for each front circuit, that move pressure around to keep the pedal height constant and to pulse the brakes.
There are isolation valves, one for each front circuit, and they reduce the amount of pulsing you feel in the pedal.
There is a mechanical spool valve for the rear circuit...

 

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Thanks for the info. This is all the non-Brembo equipment.

I can unequivocally say that the hand brake or brake pedal is not keeping pressure on it. I can roll the car easily with the hand brake off and when sitting at a light if I take my foot off the pedal the car will roll easily for the slightest amount of incline. Even in my garage with the slight incline it will roll without the hand brake on.

I get no ABS light etc on the dash EXCEPT on long drives without touching the brake (like on the interstate) I will get a solid ABS light but it resets after 2 ignition cycles. I believe this is due to having larger wheel/tires on the rear and causes the abs sensors to see a difference in RPM between the front and rear.

Well, I guess I will fire up espritmon and think I will go for a drive around the neighborhood without using the brakes and once stopped check the temps with an IR temp gauge. That should tell me if the rear are dragging or not. I really think the system is not applying force, or equal force to the front. Getting out the shop manual to plan the next attack.

Thanks again.

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The rear Bendix brakes are probably better than the rear brembo, at least for mechanical reliability.  So I'd be leaning towards a master cylinder spool valve problem, but you do need to figure out the ABS code first.  You should also check the rear proportioning valve and the rear brake lines before anything expensive like master cylinder replacement.

You could connect a brake pressure gauge to the rear caliper (in place of the bleed nipple) to see if pressure is held after release of the pedal.  That would definitely pointo the master cylinder.  Unless all this is only happening after the ABS in engaged, then it should point to the pressure release solenoid.

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Ok, after a slow drive through the neighborhood with 2 slight stops, the rear rotors were 30f hotter than the front (60f front 90f rear). Then after a spirited stop, 110f front and 180f rear. I swear there seems to be no parasitic drag coming from the brakes when the ebrake is released and no pressure on the brake pedal. I guess the rears could be ever so slightly rubbing but I cant find any evidence of that. I suppose I could disconnect the rear ebrake cables and ensure the arm is all the way released and test some more.

Otherwise, after having looked over the shop manual, I don't think the Pressure Proportioning Valve could be in play. The way I read it, it only attenuates pressures over 70 bar when applying the brakes with the vehicle stationary. (correct me if you think there is another condition that this could be causing the problem). So if that is correct, then possibly this is related to the spool valve. However, if that was stuck one way or another, it seems it would prevent either the front or rear brakes from working at all. (again, correct me if I am wrong)

Now I have to ponder if this could be related to needing to bleed the front brakes? I mean, if the fronts did need bleeding, it wouldn't affect the rear performance would it? They use separate pressures/supplies according to the manual. I am wondering if the last time I changed the front pads I let some air in the bleed nipple while having it open and retracting the pistons. Hard to think I did it on both front calipers though.

So other than disconnecting the ebrake, the only other thing that comes to mind is connecting a pressure gauge to the bleed nipple as you said on each caliper to test however it seems obvious that the rear are getting more pressure.

Just to state the obvious, no leaks and good fluid level.

Sigh....

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Quick experiment, disconnect the ABS wheel sensors to disable the ABS system.

Do a drive to see if you still have the issue. Maybe the ABS is confused or the sensors are dirty.

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I can do that but I don't understand how that could cause this issue. Do you mean that the ABS could be applying brake to the rears while driving because of dirty sensors? Unless the rears happen to be the same amount of dirty, it would just throw an ABS light wouldn't it? My calipers/hubs etc are pretty darn clean BTW. I am not dismissing your suggestion, just curious on the thought process.

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Just a quick thought, do you have matching pads front and rear? Differently sintered pads have provide more or less friction (grip) higher friction higher heat.

The fronts should get hotter than the rears as braking bias is generally towards the front.

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I was running reds on both but because the rears wore out so fast the last time, I figured I would try yellows on the back this time. No noticeable difference.

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Could be cause of the heat issue. I think I'd replace the rears with the red pads, check the condition of the rear discs for signs of heavy wear or cracking, replace the fluid and re-bleed the entire system. If this doesn't solve the issue at least you'll know you have a good starting base to troubleshoot other possible issues. Good luck.

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Just to be clear, this problem started while running reds on both. I just replaced the pads about 400 miles ago thinking that the only issue was the extra dust so I went with yellows hoping for less dust and longer life. Then just a couple days ago realized there is something else amiss.

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With the ABS disabled and threshold braking on a bend (pick a safe spot) do the rears tend to throw the car too readily into oversteer. If so your front calipers may be seized on the slides or so. Check their pads for taper and the discs surfaces, inner and outer, for even wear.

Also does your ABS work properly? You'll definitely feel and hear it.

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Thanks for the suggestion. I will see if I can find a place to give that a try. From what I can tell, when braking hard, the front seem to be braking fine. I tested today causing the ABS to kick in slightly and it "felt" like I was getting the pulsing from the front as well. Pending any other "eurekas" I will plan on removing the front wheel, checking the slides, pads etc and trying to bleed them.

Thanks for all the help guys. This one has me stumped.

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Hi John, I've had a quick look at the EBC website, while both red and yellow pads are suitable for your Esprit, the yellow rears you are currently running would actually increase bias towards the rear, disperse heat faster and create more brake dust. These being the symptoms you are experiencing, it may be worth matching the pads to put your mind at rest. 

That said, it doesn't explain the high wear rate, though as a fellow Esprit owner, I for one am often guilty of driving for a few miles without having fully released the handbrake.:sofa:

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Your pedal controls the front brakes, it does not control the rear brakes, the ABS system alone controls the rear hydraulic circuit. So if the fronts needed bleeding, you should feel a soft pedal.

Quote

The Powermaster IIIA hydraulic brake control assembly, incorporates a master cylinder operated by
the brake pedal in the conventional manner, with power assistance provided by a pressurised supply of
hydraulic fluid from a separately mounted electric pump and accumulator. Each front brake is equipped
with a separate hydraulic circuit fed from a common master cylinder chamber. The rear brake circuit is
not operated by the master cylinder directly, but uses the separate pressurised hydraulic supply also used
for the power assistance and ABS system. The pressure in the rear circuit is regulated by a spool valve
within the master cylinder before feeding a single hydraulic line which then splits to feed each rear brake
caliper.
On cars with Bendix rear calipers, a pressure proportioning valve is used at the rear circuit split
point in order to prevent over adjustment of the rear pads if subjected to extreme pressure.
An ABS controller (microprocessor) uses a wheel speed sensor mounted at each of the four wheel
hubs
to continuously monitor wheel speeds when braking. If the deceleration of any wheel is too great,
or its speed relative to the other three is too low, the controller will cycle a solenoid valve in that wheel
brake’s hydraulic circuit to reduce pressure and prevent wheel lock. The ‘three channel, independent
front, select low rear’ system uses one solenoid valve in each of the two front circuits, and a single
solenoid for the combined rear circuit, with the rear solenoid reacting to the behaviour of the rear wheel
with least grip.

No I don't think the proportioning valve is a problem, but if it were corroded or something, then it could send too much braking force to the rear.

If a single rear wheel sensor was detected as "locking up", lets say the sensor was corroded, dirty, or loose, and the wheel speed signal was weak, then the ABS would possibly activate for that circuit. If the pressure release solenoid were faulty, then the rear circuit could be energized with excess pressure for an extended amount of time.  You need to see if you can read any codes stored in the ABS system with Espritmon.

 

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Put 80 miles on today with Espritmon running. Approx 60 miles at interstate speeds without hitting the brakes (usually trips my ABS light due to different size wheels/different speed sensors readings) and 20 in city driving. Even engaged the ABS with a hard stop (confirmed by Espritmon). No codes when I connected and no codes during the drive.

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The bleeding procedure for the rear brakes is quite different from the fronts. Requires rear brake pressure on and I'd have to read it all again to quote here, but it can be easily done improperly. That said, it sound like the fronts aren't working in proportion to the rear, so maybe bleed front again? You have the same Kelsey-Hayes system I believe with having to pump the pedal a million times to release accumulator pressure, then check fluid level, etc. Overly complex 90's era ABS system, but seems to work fine in the wet -  but not very modern. 

I ran EBC at first, but Travis recommended Hawk and been with them since, they work great.

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I am going to pull the front calipers and see if the pads might be glazed etc but figured I would replace the pads while I have them off. But, I am having a heck of a time finding the EBC yellow stuff pads # for the non-Brembo brakes. Anyone have the part # for these? Thanks

Edited by Techspy

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Subscribed.

Keep us informed of your progress.  I think I'm having a similar issue with my Brembos. I've replaced the rear calipers and installed EBC Yellow Stuff to try to fix it, but it is still having more rear bias than front I think.  I should probably replace my front pads too to see if it makes a difference.

 

Jim Stone

1995 S4s

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Yep. I am thinking I should start with the simple stuff and replace the front pads. From everything I can tell, all calipers are working correctly, grabbing when braking and disengaging completely when not. Leads me to believe that the actual braking forces are equal but friction is lower on the front = pads. If I can just locate the correct yellow stuff part number for the fronts :blush:

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for the Toyota brakes https://www.shopebcbrakes.com/EBC-Yellowstuff-Brakes/p99672?sku=

Part#:  DP4456R
Product:  Yellowstuff Street And Track
Position:  Front; Pad Set
For:  (1989 - 1989) LOTUS ESPRIT TURBO

and

Part#:  DP4189R
Product:  Yellowstuff Street And Track
Position:  Rear; Pad Set
For:  (1989 - 1989) LOTUS ESPRIT TURBO L4 - 2.2L

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Thanks. I saw similar on the cross reference parts list but thought there was a difference being they model year indicated 88-89.

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The Stevens re-model used the same rotors and calipers until the changeover to Brembos. The only earlier change to that was when the Delco ABS system replaced the conventional booster in '91 or thereabouts.

I posted a few months ago about my success with getting the Delco working properly. Still does. I think a lot of the problems are related to aged accumulators and improper bleeding procedures. There's one bleed nipple on the master which is near impossible to access. Consequently I'm now thinking it's deliberately overlooked when bleeds are done. You know, the "near enough good enough" attitude.

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