Brake problem

[13B]

What method are you using to bleed them?

[oscar]

Sam stayed at home today sick, so he was the pedal pusher whilst I was calling out the up and down at each caliper.  I got about 100ml's plus from the rears each time.  Less from the fronts.  I started at left rear, then right rear, left front, right front.  I kept the pedal pusher going until there was no hint of the finest of bubbles.  Basically until I couldn't see the fluid moving in the clear tube.

I went out to the 350 just now to compare and to check I wasn't being too pedantic about povo's brakes but there's definitely a difference.  Surely there can't be that much give in the calipers.  I'll try another set of pads before the night's out.

[13B]

Call my method stupid, everyone does, because its just so simple, but its worked for 20 years on all different makes of car... it just takes 1 8mm spanner, no hose, no bottle... but it does take a little practice to get it right.

Get you helper to pump the pedal three times and hold it, open the bleed nipple and close it straight away, before the pedal its the floor.

The first two times aerated fluid will come out, on the third or fourth time just pure fluid will come out.  So tighten up the bleeder.  In this process you'll only lose about 50ml of fluid per wheel but it'll make a mess, so hit it with a garden hose and move on to the next wheel. 

I again used the same process with I changed the brakes on the turbo 190E, and again it was bled properly 1st time with less than 100ml fluid lost - I only had to slightly top up the reservoir at the end.

I.

[s class]

Check the one way vacuum valve from the engine to the vacuum booster.

I know this is a bit off topic, but if there is a problem with this valve, what symptoms will one notice?

[craigb]

I know this is probably a stupid suggestion, but on another forum talking about this huge mistake of my youth saved someone else. Are the bleed nipples at the top of the caliper? I made the mistake of rebuilding both calipers on a car and then installing them on the wrong sides and hence with bleed nipples at the bottom and making it impossible to bleed with the air pocket it left. On high school student income i paid a mechanic to diagnose the master cylinder - rebuilt that and eventually used logic to work it out for myself!

I had similar issues with my W116 when sprinting but I am sure it was to do with the standard pads not being up to the job (this is the bit where Ian should say "i told you so!") and I replaced the master in the process with a new one but no diff. Also ran motul 600 fluid.

So my advice would be not to muck about with the old lines and do what you need to replace the rears. I had no probs with the standard ate ones from autohaus, only had probs when hot and then I could bleed the pedal back easily, only to lose a couple of laps in, when I guess the pad temp so high it would boil the fluid in the caliper again.

You can get clamps specifically designed for blocking off flex lines (eg for changing a caliper). I have seen people use a vicegrip, but I would be wary of hose damage but if you are pretty sure you will change them, maybe it is worth a try if it helps to diagnose the problem.

If your not losing fluid, I wouldn't suspect a caliper, particularly if rebuilt. Something is expanding or fluid can be moving about internally in the master (as it did in the datsun recently) but you have eliminated that one or of course air somewhere. Our Rodeo was a pig for getting air out, but I haven't had any problem with the W116. To speed up the process at sprints when bleeding, I made up (i think from a recommendation on here) an old master cap, cut a valve out of an old tube, drill a hole and fit to the cap, connect an airline with a tyre inflator/pump connector on the other end. Then deflate a spare tyre to about 10psi, put the cap on the master and connect to the tyre and then the system is pressured. Then just undo the bleed nipples from longest to shortest - very fast way of bleeding.

I have some recent info about pads too, but will put in another post.

Isn't it a bastard when you get problems like this that aren't clear!

[oscar]

I'll have to try a bleed again later tomorrow 13B.  Did a bit of fiddling not long ago with the blank fittings and still found that the rears alone will be rock solid.  However the fronts alone, either both circuits or just one would introduce the sinking pedal.  Both fronts together were worse than one front alone.  I haven't got any thicker pads around but I still put some others in  in case the old pads were soft, but they made no difference.  I thought I'd try packing the pads out thereby limiting piston movement to virtually nothing.  Surprisingly there was still the same amount of sinking pedal.  So new pads aren't going to make a difference to this problem unfortunately. 

Hey Craig, I remember your posts on the master cylinders and it's only now after doing all this that what you posted before is all of a sudden relevant for me so far as me seeing the difference between reservoirs, master cylinders and fittings and problems encountered with swapping from stepped to flat barrel or vice versa.   Example; the long dual circuit reservoir can't be fitted to a stepped master cylinder which is what I think you were trying to implement IIRC.   Funny thing is that Povo has two electrical plugs for the low fluid indicators.  This means that povo used to have the flat barrel non stepped type of early master cylinder, but the steel lines must have been changed to fit a presumably replacement stepped master cylinder.  BTW I like your bleed method and IIRC sclass had a good system for a one man bleed too.  I'll keep that trick in mind.

So here's a summary - booster's fine, master cylinder's fine, rear circuit is fine and presumably the new proper hoses I have in the front now are fine.  That leaves something up with the calipers or still a poor bleed process.  I would like to block the lines right off at the calipers just to see if everything between master cylinder and pre caliper is definitely ok.  I've seen those clamps for brake lines but I don't fancy crimping my new rubber lines like that.  I'll try and get threaded caps tomorrow, failing that, some clamps.  BTW, bleed nipples are facing up.

Check the one way vacuum valve from the engine to the vacuum booster.

I know this is a bit off topic, but if there is a problem with this valve, what symptoms will one notice?

Good point sclass.  Here's my take.  At idle, no symptoms.  So long as there is no leak the vacuum will be stable and the brakes would feel normal, but as soon as you accelerate and the plenum vacuum drops, so does the booster vacuum and the vacuum assist diminishes.  Though it should build quickly once you close the throttle again.  Furthermore, after shutting off the engine, for a normal brake system but with faulty one way valve, your brake pedal would be rock hard almost immediately.

A bit more though probably over the top
My understanding after asking 13B not long ago about turbos and the effect on braking is that the one way valve helps the booster retain a reserve of high neg pressure to assist braking at all times including when there is low or no vacuum from the plenum.  Examples might include emergency stopping after WOT or high acceleration, or during if you're a left foot braker. 

Take an extreme example such as in a turboed car with positive pressure in the plenum.  If this one way valve wasn't there or faulty and you tried to brake during or immediately after WOT or during or after any boost period from the turbo,  you could imagine there'd be no vacuum assist until the the plenum returns to a high vacuum state.

[koan]

Sounds exactly like my problem or possibly a bit worse

Replaced everything, including disks and hoses, expecting a rock hard pedal but soft and soggy, pumped litres of fluid through but no good.

Had the master cylinder rebuilt, no improvement, bought a new master cylinder, still crap.

I've bled the brakes several times since expecting a miracle but no.

Four brake line clamps for rubber hoses cost about $20 and really help isolate what's going on.

I traced my problem to the front right caliper and to a lesser extent the front left. It looks like the pistons and pads draw back about a half millimeter or so when the pedal is released.

Not sure why, it's not the master cylinder, if I undo the bleed nipple with the pedal down same thing happens, the pads pull back. It might be the seals are a bit oversize and grip the piston a bit tight and flex rather than allow the piston to slide, the pistons were hard to get into the new seals. I have two new sets of seals to try again - one day.

If you work it out let me know.

koan

[13B]

Sounds like Koan might be onto something here.  Before going any further I'd try a pair of front calipers from the brown or red car, and work backwards from there.

I.

[13B]

Or even just replace one caliper and put the other front caliper off-line with a dummy plug and see if you have a hard or soft pedal.

I.

[oscar]

Yeah that is interesting koan and as much as I didn't want to swap calipers 13B it's probably going to happen after all.   

First I'll have to observe if the pistons are retracting in the rekitted ones and do a couple more things.  I thought I had today off but had to work and found myself in Wagga where I spoke to our workshop mechanics about the problem.  They told me a few of their experiences with temperemental brake systems.  I was told to give the brake hose clamps a go so I bought a couple whilst down there.  I should have bought 4 I suppose now but with a bung in the rear circuit I should be able to identify exactly where the issue lies ie, in lines or calipers of the front.   The guys at the workshop said there's still a possibility that there's an air lock trapped behind the pistons so if the calipers are the issue I'll at least try taking them off and sitting them at an angle whilst bleeding them to see if that changes anything.  Must remember to avoid popping the pistons out.

[TJ 450]

First I'll have to observe if the pistons are retracting in the rekitted ones and do a couple more things. 

Very interesting.

I bled the brakes on my car several times today and I think my problem actually is with the front caliper pistons not retracting (low pedal, but firm now). They are re-sealed Bendix units, so that's a bit of a PITA. Because the pedal is firm, but low, it makes me think that the master cylinder is OK on my car.

Is your pedal actually spongy all the way down, or does it firm up before it hits the floor?

Tim

[oscar]

Yeah, they're firm right at the end but requires a fair bit of pressure still.  I'd say they're spongy on the way.  Koan's right about those hose clamps though.  I know the rears were fine so I clamped the front hoses towards the caliper end to find the pedal was very firm after 5cm movement, no engine running.  The problem lies within the calipers.

I then released one clamp, tested, then replaced and removed the other clamp and tested.  It would seem the same as Koan whereby the right hand side caliper has more give, but either one gave a spongy pedal with more travel and combined was the worst.  On closer inspection I found that the pistons are indeed retracting when the pedal is lifted. 

I had time for another bleed of one caliper, the right front.  I took it off the wheel carrier, suspended it so the bleed vlave was the highest point, packed the caliper with pads and got the pedal pusher to put all their wegiht on the pedal but no air came out after a few goes.  I reckon if there was somehow some air still in there the pistons wouldn't retract because it would be easier for the air to expand when the pedal was realeased rather than the pistons to move.

I was a bit late getting home tonight to retrieve a couple of other spare calipers to test but it is clear that there's perhaps a couple of mm of piston retraction when the pedal is lifted.  So why is it happening?  It had me thinking about the grease I used.  Castrol red rubber grease.  Maybe I used too much or too little.  The shop I bought the hose clamps off sold the PBR grease that a few have recommended but I think you used that too koan? I think the only way to get these working properly is to take the pistons out, clean the pots and pistons then regrease with PBR perhaps. 

[koan]

It would seem the same as Koan whereby the right hand side caliper has more give, but either one gave a spongy pedal with more travel and combined was the worst.  On closer inspection I found that the pistons are indeed retracting when the pedal is lifted. 

I've worked on a few cars with discs and never noticed the pistons pull back on any of them but possibly not been looking for it, is it normal?

I reckon if there was somehow some air still in there the pistons wouldn't retract because it would be easier for the air to expand when the pedal was realeased rather than the pistons to move.

I'd agree with that.

I was a bit late getting home tonight to retrieve a couple of other spare calipers to test but it is clear that there's perhaps a couple of mm of piston retraction when the pedal is lifted.

That sounds a bit more than mine pull back.  Do they still pull back if you undo the bleed nipple while Sam keeps pressure on the pedal?  If they don't it could be the master cylinder drawing them back instead of letting more "makeup" fluid into the brake circuit (recuperating I think its called).

Castrol red rubber grease.  Maybe I used too much or too little.  The shop I bought the hose clamps off sold the PBR grease that a few have recommended but I think you used that too koan?

On the metal to metal areas I used CRC sta-lube synthetic grease which is intended for the job. I only ever use brake fluid for the seals when assembling. I've gone off Castrol red rubber grease, I revisited the the rear roll bar bushes which were lubed with red grease and found quite a bit of black goo came off the bar and bushings, and I'm getting creaking and groaning noises again from that area.

koan

[13B]

On the Mazdas I used to work on the workshop manuals inferred that when the brake pedal was released, the disc pads only come just out of contact with the brake disc.  I've always thought they come back practically an infantessimally small distance.

I'm thinking maybe the rubber dust seals are stiff and not deforming like they should do, which is maybe producing producing a retarding force withdrawing the pistons back further than they need to go.

Hows that sound?  Plausible?

I.

[oscar]

Do they still pull back if you undo the bleed nipple while Sam keeps pressure on the pedal?  If they don't it could be the master cylinder drawing them back instead of letting more "makeup" fluid into the brake circuit (recuperating I think its called).

Yeah they do pull back still.  I didn't think they were going to.  I really thought the master cylinder must be pulling fluid back, therefore sucking the pistons back a little.  But now it would appear that something similar to what 13B suggests is what is happening. 

Actually, I just went and picked the dust seals and heat shields off the front right caliper, put a pair of pads in and voila.  You're right 13B.  I had my doubts and I'm still dubfounded how these things could pull the pistons in because they feel very new and flexible still.  Put it this way, with dust seals on and brake released, it was easy to turn the hub.  Not free wheeling but easy. Without the dust seals in, it's much harder to turn the hub and it's reminiscent of other rotors I've turned by hand on cars with good brakes whereby when the brake pedal is released, the pads only just let go like what you said in your last post 13B.

I did that test again koan that's quoted without the dust seals and the pistons barely moved, just released pressure is all.  So there's the answer but for a fix, somehow I've not quite got right the fitment of the dust seal a few months ago when I rekitted these calipers.  I remember scratching my head over how it fits into the outer groove of the piston but it was too hard to describe or take a photo of so I left it and fitted them to how I thought they looked right. 

Furthermore, some food for thought.  There's a bit of grease in the dust seals as well.  I greased everything with a smear - all seals and metal parts, with that red rubber grease, nothing with brake fluid.  But there's clearly too much behind the dust seals.  Maybe that area should remain dry. Maybe that volume of grease is supposed to be air because as the pistons extend, this area behind the dust seal is probably air tight and forms a bigger vacuum that causes the pistons to be pulled in as the pedal is released.  If there was more air and less grease the vacuum generated would be overall less. Time to buy new kits anyway and read the instructions properly.

FWIW that red grease also turned brown behind the dust seals.  Probably cooked.