Brake pedal mushy

Started by MrChris, January 5, 2023, 14:53

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MrChris

Hi all,

Another post about a mushy brake pedal... I've had 2 new rear calipers, fronts have been refurbished with new pistons and seals, I have bled the brakes to buggery. Literally no air bubbles are coming out and I've probably used well over 3 litres of fluid now. I've got Yellowstuff pads on the front with plenty of life left and Apec pads on the back, also with plenty of life left.

When I drive the car, there is an initial mushyness to the pedal. It goes down and I start to hear the rear brakes grab but I'm not braking until the pedal is well down its travel. If I quickly pump the brakes, a firmness that I expect seems to come in.

I have also noticed under heavy braking that the ABS is triggering way more than I'd expect it to.

Other things:
I have no loss of brake fluid.
The brake fluid is quality and new.
My handbrake seems fine.
When the car is off there is still a bit of initial play in the pedal before it goes firm. It is still firmer as you'd expect with the car off.

Any ideas or suggestions would be really welcome at this point. I've got a track day in a couple of weeks and I need confidence in the brakes.

Edit: Wondering if it could be that rear discs are worn too much? @shnazzle I know these are potentially quite old? Other thought is ABS module is on its way out... but it is triggering so perhaps not?

JB21

Potential issues:

Pads are compressing
Air in the ABS pump
Master cylinder is drawing in air
Brake servo is on its way out
Leak on the brake servo line
Calipers haven't been rebuilt properly

Carolyn

If it pumps up, it's got air in it.
Perry Byrnes Memorial Award 2016, 2018.  Love this club. 
https://www.mr2roc.org/forum/index.php?topic=63866.0

threepot

You could eliminate some things above (caliper related), by putting a brake hose clamp on flexi hoses. Then you can ensure it isn't a caliper issue.

I doubt it could be the servo/diaphram as the pedal would at least be consistent. But a fucked master cylinder is possible.

Have you tried bleeding it with the engine running so the ABS module is powered up ? I had a very similar problem with my dads RAV4 about a year ago after doing new brake lines to the back of the car. I think some of the valve block or accumulator doesn't bleed properly unless it is powered up. This is a blind stab in the dark. But he had failed to get the bleed worthy after quite a while... then I started it up and he pressed the pedal while I did the bleed nipples, and within minutes the pedal was bang on.

mr2garageswindon

Check for sticky brake sliders on the front. Rear pad nipple is in the recess too I take it?

Ardent

Quote from: threepot on January  5, 2023, 16:44Have you tried bleeding it with the engine running so the ABS module is powered up ? I had a very similar problem with my dads RAV4 about a year ago after doing new brake lines to the back of the car. I think some of the valve block or accumulator doesn't bleed properly unless it is powered up. This is a blind stab in the dark. But he had failed to get the bleed worthy after quite a while... then I started it up and he pressed the pedal while I did the bleed nipples, and within minutes the pedal was bang on.
I can see me trying that. I have forgotten what my own thread was. But would just like that bit more firmness right from the off.
If I want it to stop, it stops. But I wonder some times if it was designed that way. Initial soft and gentle for 80% of the time. Day to day use. But will have your nose pressed against the screen if you really ask.

As an amateur driveway spanner spinner. I will have to jack and remove each wheel in sequence. I take it, it should not matter if the engine is off between each wheel. Just so long as its running at the time of bleeding.

Dev

#6
I can think of two things.
 1. Trapped air behind the pistons that need to find their way to the bleeder screws. It happens in a very dry system that is not bench bled. Sometimes tapping it both the master and calipers might shake it loose.

 2. I did replace one caliper on a previous car and could never get the pedal tight as before. Upon further reading from numerous complaints some of these caliper manufactures use grease in the assembly that create this condition from what I read. I do not know how since grease along with the fluid is non compressible but anything is possible. Over time I noticed that the brake fluid would get dirty and it was due to whatever grease was added.
 In addition to this a local member replaced all of his calipers with refurbish ones from the same company I bought and experienced a worse situation where no amount of bleeding fixed it.
 I do not know the reason but I would suspect the remans.
 
After that experience I decided that if I ever need a caliper I would buy one from a low millage wrecker or new OEM.






 

MrChris

#7
Thank you everyone for the suggestions.

I've gone round, tapped on everything, re bled again with engine off and on. All calipers are moving freely, I even put new rear pads in.

Result: a slightly improved brake pedal, which I think is from the new pads. However there is still an initial mushyness/dead travel. The pedal hardens quicker, it takes one pump, not two.

Unless there is *still* air in the system I'm thinking master cylinder on its way out?

I'm loathe to take it to a garage as I've read of stories where they're unable to sort it. Plus I like to do all the work myself. But this may be a last resort...

Oh yes and I hope I'm not going to need a full set of calipers. The problem was there before I refurbished the fronts anyway. It appeared after a track day when everything was working fine before.

shnazzle

Have you replaced all your fluid?
Just wondering whether you've boiled your fluid, in which case a full flush is the only way forward. As you said it was after a track day
...neutiquam erro.

MrChris

Quote from: shnazzle on January  7, 2023, 15:28Have you replaced all your fluid?
Just wondering whether you've boiled your fluid, in which case a full flush is the only way forward. As you said it was after a track day

Replaced several times  :)

shnazzle

Quote from: MrChris on January  7, 2023, 15:36Replaced several times  :)
That's not it then.
Is there a hissing sound when you press the brake?
Wondering if it's the brake booster. Significantly cheaper than pump
...neutiquam erro.

threepot

Also helps if you get the rear of the car 2 foot higher than the front.

Bet your not bleeding those long pipe runs out.

threepot

It also helps if you concentrate your efforts to one braking circuit at a time.

I do not know the plumbing/internal abs config for the MR2, but typically they work with opposing pairs from each master cylinder circuit - e.g. front left + rear right ---- front right + rear left.

So you want to just bleed one circuit at a time, and keep in mind the rears always require far more purging due to the pipe length.

You need to be quick on the spanner.... shout pedal down, open the valve, close the valve.... shout pedal up... get it done 15 times in quick succession so the air doesn't have chance to go back to the high spot.

And also always bleed out the master cylinder first by doing the bleed by opening the flare nuts on the ABS module that come from the master cylinder. Give them a few pump purge outs first, then work your way down the path.

I've been in the diagnostic game for over a decade, I've had countless cars dumped on me with a suspected abs module fault etc from garages, but its just a bleed up problem, normally after someones been doing brake lines or major overhaul.

All cars do not work the same - either they are plumbed in pairs for each axle, OR opposing corners. Here is a diagram to show a typical Bosch setup. Most I've done are opposing pairs. Its only cars with LSPV on the rear axle that tend to be a bit weird, and are mega hard to bleed.




But all of this is pointless if there is something goofy with the pads sitting wonky, or the rear pads don't have the peg in the cross properly, or you have a leak etc.

Joesson

Seldom (if ever mentioned) in relation to car / brake hydraulic systems but air can be entrained in the brake fluid if carelessly poured into the reservoir.
As said, not read of it to do with car systems but larger hydraulic systems where oil is poured from eg 25L drums into a reservoir air can certainly get entrained and is best left to settle out before proceeding.
When all else fails ?

MrChris

#14
Quote from: threepot on January  7, 2023, 16:53I've been in the diagnostic game for over a decade


Thanks @threepot I like what you're saying here. And as @Carolyn says if pumps then there's air. I guess there's still air. I will try and bleed the master cylinder per the Toyota manual, I'll get the rear end as high as possible and do another bleed. As I said, it's definitely better so it must just be a trapped bubble or something.

Quote from: shnazzle on January  7, 2023, 16:05That's not it then.
Is there a hissing sound when you press the brake?
Wondering if it's the brake booster. Significantly cheaper than pump

Definitely not the booster, at least none of the symptoms when I google/YouTube the issues associated with brake boosters i.e. no hissing etc.

Ardent

Quote from: threepot on January  7, 2023, 16:53And also always bleed out the master cylinder first by doing the bleed by opening the flare nuts on the ABS module that come from the master cylinder. Give them a few pump purge outs first, then work your way down the path.
I take it by using this option. You can do it yourself without need for a 2nd person. Just a rag to catch the fluid.
Vs, the manual suggested way of disconnecting the pipes at the master cylinder end and covering the holes with your fingers in sequence as someone else operates the pedal.

threepot

I don't ever disconnect anything, I just uncrack the flair nuts 1 turn and thus it leaks. Use them just like bleed nipples. I never do the finger block method, always open/close the nipple/flare with a spanner.

If you ever have anything open while the pedal goes up, you're going backward by sucking air back in.

There has to be firm strict communication between the pedal pusher and the spanner guy, no chitter chatter or mobile phone distractions etc:-

I tend to do the spanner/nipples which is the lead job...

I shout "down", the pedal guy does what I ask and shouts "down" to confirm....

I shout "up", the pedal guy get the pedal up and shouts "up". You can get a fast accurate rhythm going.

Some times I shout "pump it", and then they tell me how the pedal feels.

Some times I shout "press and fall"... e.g. press the pedal very hard, then I open the nipple and let the pedal fall to the floor.... then they shout down when it hits the floor.

Pedal guy gets a worn out leg! Also if possible I put a mirror on the master cylinder reservoir so the fluid level is watched by pedal guy... as it gets lower they tell me and I top it up.

Joesson



As mentioned previously,I first bled brakes in the early '60's, first with a jar and tube, then with automatic valves but then with
Automatic Brake or Clutch Bleed Tube [PWN189] 25cm Long Rubber Bleeding tube.
Many company cars later and the need to bleed brakes for myself again I bought another.
Saves me needing to shout at Mrs J. Neither method has ever failed for me, but the split tube is a very simple device, it has been available to buy, to my knowledge for 60 years, so, just maybe it has some credentials.

MrChris

Hi all, thought I'd update on this more out of courtesy than anything.

The brake pedal is "good enough" for now. I've basically got very little time to get out and bleed the brakes, but I consider them to be fine. I'll hopefully get a chance to do another bleed before my next track day where I'll look to also bleed the ABS unit as per @threepot  recommendation above.

As ever I believe @Carolyn was originally on the money. It has to just be air in the system. I can't see any symptoms to do with booster or the master cylinder (there's no way I have the time to swap this out to check though).

Strangely as the car has sat for a few days the pedal seems slightly better... maybe this is air working its way to somewhere it's not as noticeable?

Anyway, the saga will continue I'm sure.

Joesson

@Mr Chris
Thank you for the update.