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Brake issues with ABS booster


 
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ZambiaTraveller
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Joined: 12 Dec 2019
Posts: 1
Location: Banbury

PostPosted: Thu Dec 12, 2019 8:36    Post subject: Brake issues with ABS booster Reply with quote

Hi all,

First post and sorry if this is a re-hash of previous advice supplied.  To complicate matters I exported my 120 ('05 model) to Zambia when I moved there and the local mechanics may be the source of my present issues.  I am reasonably technical but usually let my own workshop guys loose on my vehicles.  Possibly a mistake here!
In short immediately after some recent work which included a change of rear pads the brakes started to feel spongy.  On checking and bleeding the rears were fine but the fronts had no fluid.  Initial diagnosis by local guys told me I needed a new booster - their term for the ABS assembly - and a quote from Toyota was obtained at around $3,000 for the part. Ouch.  This is where I got involved.

I called in local Toyota agent with their diagnostic system and they said the problem was likely to be solved by bleeding in the right order, using their control system.  This was done and all seemed right until a road test where the fronts again quickly lost fluid.  There the interest of Toyota ended and they now agreed that the "booster" needed changing due to "failure of internal seals".  I am not happy with this diagnosis (not least because of the $3k price tag).

I have not found any schematics of the system but my knowledge suggests that there is a pump, an pressure accumulator and the valve body with circuits activated by the ECU.  I really don't want to start dismantling the whole if there is a more simple route (such as bleeding the pump/accumulator unit - I have a sneaking suspicion that when bleeding the rears on changing the pads that the master reservoir may have run dry and there is air in the whole system - but maybe I am over-simplifying things).

Any help gratefully received!

Cheers,
Marcus
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Juddian
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Joined: 31 Jan 2015
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PostPosted: Thu Dec 12, 2019 11:33    Post subject: Reply with quote

Up until i got the 120 i have always changed the brake fluid and bled the brakes on my own cars, but this Toyota system is different and i will leave bleeding until the vehicle needs to go into Toyota for some reason, when they can do it.

There is much can be damaged by not bleeding via the correct procedure i am led to believe.

When pushing the pistons back in for fitting new pads is when damage can occur too, its possible to 'flip' seals in the master cylinder doing so, this has never happened to me in some 50 years working on cars, but i have always been gentle in the pushing the pistons back (and probably lucky too), some people say you should open the bleed valve on the caliper in question when pushing the pistons back in i have only done this a couple of times as it's possible to get air in also imho unless you have an extra pair of hands to lock the valve closed again whilst the piston is still going in.

I do not know if your problem lies in the master cylinder or the abs unit itself, or is the booster pump failing...something which can be rebuilt by at least one workshops in London, don't ask me where you'd find such a place in Zambia.

Personally i'm not that keen on the Toyota system, yes its brilliant whilst working but at what cost when it fails (Mercedes went down a not dissimilar route with SBS, which they dumped in short order), i'd sooner a traditional vacuum pump, servo canister and separate ABS unit, its not as if there's a lack of room under the bonnet, Toyota got a bit too clever with this and it's a niggling worry for many of us wondering when this stupidly expensive ticking time bomb is going to go off.

So firstly check the same lack of fluid is happening without the brake booster operating to rule that out, i doubt there's an issue with the front calipers but before you do anything at all check you haven't got partially seizing front caliper pistons, which are suddenly moving under brake pressure and fooling everyone due to air being trapped inside which then finds its way back upstream, seizing pistons and/or sliding pins is a favourite for Japanese cars wholesale.
If the pistons are partially seized they might be staying in place whilst bleeding but then moving under braking pressure.

You could strip the master cylinder out and check and hopefully replace all the seals inside, whether the Toyota dealer will sell you a refurb kit is anyones guess, but a decent motor parts shop should be able to locate the right seals from somewhere even if supposedly for another vehicle entirely but this will be time consuming, i know because i've been there before with unusual vehicles which had no seal kits available...obviously don't dismantle until you have located someone who can supply various seals, somehow i doubt there is a specialist brake refurb shop in Zambia but you never know, though i imagine mechanics there are more used to fixing things economically than just throwing new parts at the problem which is what they do in the west.

A used complete unit out of wrecked vehicle might be a better bet, but i can't stress how important it is to rule out those front calipers first (and recheck the work done so far on the rears, ie are the pistons free moving)...i had issues once years ago trying to bleed an ex army Landrover with drum brakes all round, with the slave cylinders constantly moving during bleed operations not allowing a decent bleed out, adjusting the shoes up tight didn't help so i ended up removing all brake drums and shoes and clamping the slave cylinders tightly closed with G clamps (could have wired them shut too), that worked and it could in theory be used in this case, by bleeding the calipers whilst the pistons are held fully in their bores by wiring tightly or G Clamps, but hopefully it won't come to that.

Doubt any of that has been of any help, but might trigger an idea for you.
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